I love generating ideas for stories, which is nice, because it is what I have spent the last few hours doing. I've said it recently, but it's wonderful all the different things you can do with the same basic concept.
I suppose in a normal writing context, the question becomes one of what you choose to give your time to, and how much time you give it. When you look over ideas, do you know already what is going to be a short story, a poem, a piece of flash fiction or a novel? Mostly, I have a length in mind while I'm thinking, so I already know how much time we're talking.
In the case of a novel, it's months. Even years. And that time is time you could be spending on another idea. So how do you choose? How do you decide which one is worth your time? I'm told that one rule is to pick something that really excites you, and that will change you in some way just to write.
When you're working with ghostwritten stuff, the process becomes complicated by the question of how much you should get involved in the planning process. For me, that's a question whose answer is down to the client, and comes down to how much they want you involved. Giving them the best book in the world isn't any use if it isn't at least broadly the book they had in their head. The aim is simply to produce the best possible version of that. Of course, that means one or two left over ideas, which if they're sufficiently different, can go in the pool with the others, so I have an even harder time picking.