Given the amount of debris, and recent trends in launch costs, launch rates, space project completion times and budgets... I'd say decades sounds entirely reasonable. If we had a more serious launch infrastructure in place, it might be done in a few years. Right now, we might get a useful pilot result in a few years if we started a high-priority project today. It would be more years before the project was operating at a useful rate, and yet more to finish cleanup.

Part of the problem is that very small objects moving at high velocity are problematic, and there simply is no easy way to catch them without creating more such objects. At present, many serious proposals actually involve handling every piece of debris individually. That's expensive, to say the least. Other proposals involve complicated stuff with deploying large objects like thin films, threads, etc., which is an activity with a very poor success rate thus far.

So there are a few proposals to dealing with this sort of thing, but most serious proposals are more preventative in nature. There are good engineering solutions for picking up old satellites and large pieces of debris. But small pieces of debris are not as easily dealt with- there have been some proposed solutions but no one is clear on if any of them are actually viable, at most right now there are a few toy models and back of the envelope calculations that suggest some solution will work. Right now, large nets seems like a promising idea but the problem of puncturing is serious.