For copying the basic outer form - maybe 2-3 times per week.
For actually training your body to do anything worthwhile - each and every day.
I usually do an hour in the morning and an hour in the evening, if there's no group training that day.
I keep thinking of posting, but really I'd just be repeating what Mr. Li wrote above. At our dojo we have daily classes. I've seen guys come more often than 3 days a week and they seem to kinda max out at that point. And it depends on how many hours each day. And it kinda depends on where you are in your training. Honestly, truly enjoy and cherish my time on the mat now, but I spend time each and every day doing things on my own to build up the stuff insides. But that's after a bunch of years training a lot. I was one of those guys that tended to come 3+ times a week when I first started, often for multiple hours each time. And I was anal enough to be doing things like our AIki taiso by myself in my empty office late at night when everyone was gone.
It's just going to depend on where you are and where you're planning on going...