I think you can consider a "whoosh" any loss that's a greater amount than you typically lose when you haven't done anything differently.
I know I get a little whoosh following the mini-plateau that accompanies TOM. I'll lose next to nothing for a week, then within 3 days, I'll drop 4 pounds without making any changes.
I don't think there are set numbers, just an unexpected, bigger than usual drop based on your individual experience.