Good for you, I hope that juice fasting helps you as much as it's helped me.
As far as how long for your first fast, it will depend on how you react. Some people need to start with shorter fasts and work their way up to longer ones because of too much detoxing happening too fast. I've read that we're supposed to fast until our tongue is no longer coated.
I'd suggest a couple of things:
1) Make sure your bowels are moving before and especially during the fast, at least once a day. I use Barefoot's LBB for this.
2) Read, read, read so that you understand what's happening. I have a few favourites - Arnold Ehret's books, Stanley Burroughs' "Master Cleanser", and Book I, which is on MH's website. You could also check out plz's forum "Barefooters' Library" to see if she has any articles on juice fasting.
3) In my opinion, fruits are better than vegetables for fasting. Fruits are extremely cleansing. I did well with The Master Cleanse and also with the Orange Juice fast.