Part of it depends on how much you enjoy writing. I personally don't like it at all, so I avoid it whenever possible.

My writing probably peaked in late high school/early college when I had to do a lot of writing. I did well in freshman English and then proceeded to avoid anything to do with writing. By the time I was done with college, it was pretty poor. I nearly failed legal writing in law school, partially because I was so rusty and partially because I really dislike writing.

I'm pretty sure I'm forgetting the rules of grammar over time, being that I have no real interest in retaining that knowledge. It's just one of those areas of life I find incredibly boring.