We know that about 40% of the room bookings are made the same day, and about 70% are made within 3 days of the meeting, so if you can plan ahead, even just a few days, you are pretty much assured of getting something.
If your meeting is for more than 6 or 8 people, you’ll want to plan a bit further in advance because the number of rooms that can accommodate your meeting goes down from several to 1 or 2 (in most locations). If it’s an all-day meeting you’ll also want to plan ahead more than a few days as well, because a one hour meeting by another member wipes out that room as a possibility for an all-day booking.