Second this anecdote, and also the parent. I think depending on how you intend to apply it, you may be more or less observant of the short term effects of meditation, esp. mindfulness.

Anecdotally, sitting shikantaza style (just sitting, subtle attention to posture, returning from daydreaming or distraction when you're aware it's happening) has so far had two effects - short term I've found some immediate benefit in terms of being able to apply attention (which was a goal for me), over the span of 1-2 years I've (subjectively) noticed changes in how I react and make decisions which have been re-enforced by other peoples observations.

Further to this I've noticed it's easier to apply some learnings, for example techniques from less wrong. Internally it feels as if there's a longer period after an event happens to internalise and construct a response, instead of an immediate / knee jerk reaction.