I always find it really hard to adhere to those practice schedules that everyone talks about. I'll just start off by goofing around, improvising, and doing some shredding, and when I look up at the clock an hour will have passed.
When you're starting, provided that you're serious about it, you'll probably want to practice at least an hour a day. And if you're having trouble switching between chords, I'd recommend learning some simple strumming-type songs.
What? . . . you're still reading this post? Why aren't you practicing??