I begin giving my bunnies a few greens as soon as they are interested in eating them, usually around 3 to 4 weeks they start trying to get nibbles when I feed their mother some greens. Organic cilantro seems to be a favorite first green and it really is not much more than a taste the first few weeks. However, a steady supply of greens we do not do; greens remain the smallest portion of their diet.
I am in agreement with Bossroo about the parasite and bacteria issues that come from having rabbits on the ground. I will not even consider buying from other breeders unless they keep their rabbits off the ground.