I'm always in that position. I've 2-5 I'm prepping for at any given point.
To keep burnout at bay when I am running a campaign I am always working on a second and sometimes third campaign.
My campaigns usually run 2+ years. If I always had something in prepping stages, I'd end up with a long series of prepped games I never used. I'd rather spend my free time doing other stuff, and wait to prep when I know I'm going to need something.
Which means I collect a stack of ideas, none of which are prepped, and I have to choose which one my players want now.