Most likely will be using spray copper gasket seal, since the gasket that came with my ASM headers are garbage and they're sealing up about as well as a sieve.

Question is, can i spray the gaskets tonight and use them tomorrow or the next day, or will i have to use them right away?

 

You can spray them today and use them tomorrow no problem. Just be sure they don't get full of crap while laying around.
However, if you are having sealing problems I don't think this will solve them. The spray goes on fairly thin and will not normally hold up to exhaust temperatures and pressures.