I was always testing them by just shooting my feet and if it damages me, then I’d know it has splash. But the spinner as far as I remember does not inflict self damage so I assumed it didn’t deal any, but the wiki says it does.
It’s on a roughly 60s timer. It starts counting as soon as you load into the game or equip blast master, and resets on reload or taking the mod off.
Has nothing to do with actually firing the gun like you’d assume. Also just an FYI Iron Bear gets the bonus damage with splash weapons as well, and the mech’s reloads don’t reset the timer.