Most of the other airlines are really nice about giving me jumpseats and I'm glad to return the favor when I can.

I can't recall ever seeing or hearing an "official" canceling cut off time but ... you might be taking a seat away from a commuting pilot who couldn't book a jumpseat because they were "full". We actually have a "Jumpseat Department." Have you called them and asked your question?

It sounds like your planning to cancel at the last minute? The courteous thing would be to cancel as far in advance as possible.

And ... Welcome aboard!