It really depends on what needs to be learned and also your age. About 10 years ago, I started learning Modern Greek for a holiday in Greece. I did quite well and reached maybe slightly above "survival" level. I could hold a short conversation about family, ask my way and understand the answer and go into a shop and purchase things by asking for them. People also understood what I was saying. Without constant practice though, that level of understanding has dropped and I need to go through a similar (though shorter) cycle of learning to get back up to that level. On the other hand, I did French in school and reckon I still remember about 80% of what I learned then, even though I didn't have the interest that I displayed with learning Greek. I also calculated that I spent roughly equal amounts of time learning French and learning Greek but at vastly different ages. I may not be as fluent at speaking and understanding French but my reading and writing level are not too bad, so your age also has an effect.
I am also doing advanced studies. I find some of the things I am learning are real "eye openers", the kind of thing you say, "I only wish I had known about that years ago". Those types of thing need only 1 cycle of learning to understand and apply, though I might read over them several times to make sure I am applying them correctly. So, even though i am a lot older, there are still some things that i can learn quickly.