Hey Micro-
It's a matter of personal preference. In my time as a percussionist, I've had opportunities to practice 4 hours a day, seven days a week and situations where I couldn't get more than about 20 minutes a day in. You have to figure out how to make the time that you have productive and fun.
I'll share a couple of my strategies. When I was practicing a lot in college, I'd do two two hour sessions a day. I'd spend the first 30 minutes warming up and then I'd practice whatever I had to do for lessons and performances. This usually involved practicing multiple instruments (i.e. snare drum, marimba and drum set).
However, in the six years that I have out of undergrad, I've been working a full time job, married, etc. I've found that I can put together 5-6 1 hour practice sessions in a good week. In a bad week, I can only put in about 20 minutes a day. Since I am mostly playing latin percussion now, I can limit these sessions to one or two instruments (i.e. bongos and congas). I've had to learn how to make these sessions effective.
My strategy here has been to warm up with about 1/4 of the time. If I'm practicing for an hour, this means that I can get good and warmed up. I use some of Gio's and Jerry G.'s exercises to warm up, and that helps me build my independance and chops at the same time. After that, I spend some time on the material from my lessons, and some time playing along with CD's. On the 20 minute days, I try to focus on my lessons.
Outside of my practice sessions, I'm constantly listening to CD's and thinking about how different players approach different situations. It is amazing to me that after listening to a tune for a while, I can just sit down and replicate a lot of what the player on the CD played. Also, I hear other players in my playing when I haven't even practiced their licks. This tells me that it was time well spent!
I hope this is helpful. The bottom line is, figure out how you're most productive or how you can adapt to the time that you have.
My $0.02,
Zach