Personally, I do about 15 minutes 5 days a week, but it's mostly comic work. I don't free sketch as much as I used to :<
I think what's more important than how much you draw, is how mindful you are. If you're doing the same ol' drawings as always, you can improve speed, but your repertoire of skills will not grow.
When you do your 10 minutes a day, try to incorporate something you find challenging, do a color study, things like that. It will help you improve and you don't have to burn out your creativity