Around 3-4 weekends. Althought being actively interested in your surrounding sounds is somewhat a big part of it and that happened between more intense sessions. I found that having and considering edge cases that are just in the limit of your perception is the most developing. I used a walk-in closet to familirise myself with the direct voice in contrast to the echo. Empty rooms are actually noisy. The drop in volume is significant enough that there is a clear difference in effort to produce equal sound even in mono mode. I also tried to have a reference sound I can produce uniformly in a variety of places and isn't disrupting to other people. One was base of my tongue against my palate. However this is a little confusing as the head internal accustics are not the most straigthforward ones and interfere with the external accoustics. I also had a button I would click into place and out of. This had trouble in that it often would have insignificant volume to get a proper feel for environment.
One most not forget about just being curious about sounds that just happen to be in the environment. Emergency vechicles are a great source of doppler and the volume output is really great. In urban areas there are plenty of clear surfaces and surface gaps making the moment have a nice variable microstructure. In more wide open spaces the scale of things makes it more easy to pick up on the echo components. Cars in general provide a pretty monotome moving sound source. Riding a bike also provides a constant mechanical noice that has relative position to you fixed and doesn't really tire you out in generating (+ is socially acceptable way of being noizy (you can even get away with devices explictly designed to generate noise (atleast if you are young enough))). I didn't really use it myself but cell phone button/ui noises should be pretty standard, narrow and somewhat acceptable.
In private areas clapping has pretty narrow sound profile althought is pretty directional that can make the volume non-standard when you haven't masterd that yet. Listening to the wall of a detached house with clapping could be done within it's yard. The smaller scale you are the higher you want the pitch to be (or can only latch into higher components).
The main thing is to be aware and ready to percieve. It's clearly a very learnable skill the main obstacle being paying that attention. I didn't use any reference or readymade learning materials. Having a goal was plenty in providing steps/structure to proceed (ie thinking that there are possibly harder and easier sounds, you focus on what could determine the easiness/hardness of a sound, having a bunch of hearing experiences focusing on what kinds of categories you can place them in can then be used to anticipate categorising novel experinces etc). You have ears, use them, play with them. Shockingly most people really don't. I have yet to generalise what other things could be achieved with "seriously playing". Being able to take into account findings "midflight" might be a critical thing that most learning alternatives lack. You won't need that many repetitions but you need to be level-appropriate (even as that level shifts).
Thoughts inspired by Yvain's philosophical role-playing post.
Thomas Nagel produced a famous philosophical thought experiment "What Is It Like to Be A Bat?" In it, he argued that the reductionist understanding of consciousness was insufficient, since there exists beings - bats - that have conscious experiences that humans cannot understand. We cannot know what "it is like to be a bat", and looking reductively at bat brains, bat neurones, or the laws of physics, cannot (allegedly) grant us any understanding of this subjective experience. Therefore there remains an unavoidable subjective component to the problem of consciousness.
I won't address this issue directly (see for instance this, on the closely related subject of qualia), but instead look at the question: suppose someone told us that they actually knew what it was like to be a bat (as well as what it was like to be a human). Call such a being a vampire, for obvious reasons. So if someone claimed they were a vampire, how would we test this?
We can't simply ask them to describe what it's like to be a bat - it's perfectly possible they know what it's like to be a bat, but cannot describe it in human terms (just as we often fail to describe certain types of experiences to those who haven't experienced them). Could we run a sort of Turing test - maybe implant the putative vampire's brain into a bat body, and see how bat-like it behaved? But, as Nagel pointed out, this could be a test of whether they know how to behave like a bat behaves, not whether they know what it's like to be a bat.
I posit that one possible solution is to use the approach laid out in my post "the flawed Turing test". We need to pay attention as to how the "vampire" got their knowledge. If the vampire is a renown expert on bat behaviour and social interactions, who is also interested in sonar and paragliding - then them functioning as a bat is weak evidence as to them actually knowing what it is like to be a bat. But suppose instead that their knowledge comes from another source - maybe the vampire is a renown brain expert, who has grappled with philosophy of mind and spent many years examining the functioning of bat brains. But, crucially, they have never seen a full living bat in the wild or in the lab, they've never watched a natural documentary on bats, they've never even seen a photo of a bat. In that case, if they behave correctly when transplanted into a bat body, then it's strong evidence of them actually understanding what it's like to be a bat.
Similarly, maybe they got their knowledge after a long conversation with another "vampire". We have the recording of the conversation, and it's all about mental states, imagery, emotional descriptions and visualisation exercises - but not about physical descriptions or bat behaviour. In that case, as above, if they can function successfully as a bat, this is evidence of them really "getting it".
In summary, we can say "that person likely knows what it is like to be a bat" if "knowing what it's like to be a bat" is the most likely explanation for what we see. If they behave exactly like a bat when in a bat body, and we know they have no prior experience that teaches them how to behave like a bat (but a lot about the bat's mental states), then we can conclude that it's likely that they genuinely know what it's like to be a bat, and are implementing this knowledge, rather than imitating behaviour.