It's been two months since the last post here, but as a new member I thought I'd throw my 2 cents in.
I'm a 'manic builder'...meaning I'm not one of those folks who ALWAYS has something ongoing. I will go weeks, months, or (unfortunately) years without building, but then will get the bug and will build furiously for a month and turn out a model a week for a while. Point being different builders have different styles of building, so I sometimes like the idea of a quick GB with a short deadline. Sometimes with a GB I will be excited about the theme, but lose a bit of the impetus when the end date is 3 months away. Anyway, this 'quick build' method is just my own personal way of enjoying the hobby.
I think 2-3 builds at a time is a good pace. Three more if they are staggered a great deal, or if one is a shorter build.
I love the idea of a "Missed Deadline GB" for the end of the year so that anyone who begins a GB project at least gets a second chance at having it in a GB, since sometimes things come up beyond our control, and even with 3 months to work at it a deadline may be missed. Seems a shame to put work in for a GB and not have a chance to enjoy the competition.