I took 3 years of piano in college music school, but that ended in 1996 and I've forgotten most of it. But I do remember that if I put in a half hour a day, I kept up very well. But if I skipped a day, then it was a pain trying to put in an hour the following day, or even worse, 90 minutes after skipping two days.

On my main instrument now, I haven't missed a day for a couple years. Most days I practice for a couple hours (spread out over several sessions throughout the day), but I always play for at least a few minutes, even on vacation or staying at my in-laws or whatever. I keep a journal of what I play every day, and if I have a good day, I know what led up to it so I can keep doing it. Likewise, it helps me avoid bad days. I think of my skills as a balloon; when I practice I bump the balloon upward, but after a few hours, it stops rising and then starts to fall. After a couple days off, it's falling fast. It's no fun trying to climb out of a hole after not having played for several days. If I find myself playing with weekend warriors for whatever reason, I realize that many of them only play one or two days a week, and so they're *always* trying to climb out of a hole. They may not even know what it feels like to have a good day.