It depends on the bunny. Was she with her mom for a full 6-8 weeks? Did her mom have veggies during the time that she was feeding the babies? How does the bunny react to stress/new food? Has the bunny ever had GI upset, and what caused it?
If she has a pretty stable GI tract, you can probably start introducing greens slowly when you want to. Start with small amounts--half a lettuce leaf, etc, and only give every other or every 3rd day. If she tolerates that well, you can increase it. Start with easily tolerated veggies, like romaine, red/green leaf lettuce, parsley, dandelion greens, spring green mix, and cilantro.