I think it is really up to the individual.

I think it is really up to the individual. Though, I would agree that you need to write a lot to improve. Or even just to keep yourself motivated enough to produce more writing.

Personally, I find writing 3-5 times a week is more than a enough for me.

Recently, I did a 50 day challenge in which I wrote 500 words a day. Most of what I wrote was pretty terrible. I'd much rather write less when the writing is really flowing than force myself to write something that feels flat and dead. Just my personal take on it though.