I hosted my first GB back in 2010 (GAF, in case your woundering, i have hosted 18 of my own and a few more as co host for other's GB's).
I think there has been some great advice here. I would deffinetly recommend joining a few GB's first before hosting your own. I have become a bit weary of joining a GB if i do not know the host that well. As others have said, there have been quite a few GB's were the host has just vanished. I know life can get in the way, but on a few occsaions i have seen the same person do it several times.
Personally, i like to plan my GB's ahead. Think of a subject, get a badge sorted, put out a feeler if i am not sure and give a month or so notice. I am not so keen on GB's where you simple have a post that says this is the GB and it starts tomorrow. But, maybe thats just me.
I have not had a single day since 2013 were i wasn't hosting a GB, and there have been a couple of times were i was hosting 3 at once. Personally, i think once you have the idea, the rules and the post, the members will make it a good GB. The host just needs to be attentive, try to check in every day, or every couple of days at most. People have made the effort to join your GB, so make the effort to make them welcome. If a GB is really busy, it is easy to miss a post of a build or an entry, so make sure people feel comfortably in pointing out if you have missed somthing.
I take the veiw that if a GB is a success, its down to the members who joined it. If its a faliure, its probably down to the host.
And yes, timing is key. As i think i put in that long post that GAF linked (i had completly forgotten about that), personally, i think around a dozen GB's at any one time is the max. Though the forum does not seem to be as busy as it was in 2013, so maybe that should be less.
There are currently over 20 open GB's if you including Checks World at war anniversary GB and Steve's ships GB's which are pretty open ended.