I'm not sure...but I "think" it's 11. Since it's a buffet they'll be changing out the breakfast items for lunch items probably starting at 11. If you tryed for ADR at 10:30 you'd be able to sample both breakfast and lunch items. And I believe (could be totally wrong) that if you book breakfast ADR but stay over until lunch is served you still only get charged the breakfast prices.
You can book up to 11 am. Also when we went, Boma didn't do lunch and we could stay until we were fininshed with everything we wanted. They left it out until everyone left, they just didn't take anymore ressies after 11 even though they don't pull the food at 11.
Someone on another thread also just said Boma dosen't do lunch. Sorry. I didn't know. I haven't ever looked into Boma for lunch...so I didn't realize. Anywho...sorry for the bit of miss information.