Plan for two to three hours for an effective session. Shorter doesn't work: your brain needs time to move past the obvious ideas. Longer doesn't work either: after three hours, sharpness fades and you start repeating what's already on the table.
At Live Impact, we work in three rounds of 25 minutes with breaks in between, plus 30 minutes for opening and closing. Beforehand, you spend at least an hour framing the brainstorming question and selecting the participants. Afterwards, you need another day to organise the output and write a concept A4.