In a recent encounter the players managed to get the last living opponent to surrender, when he proved uncooperative one player said "flick my gun to stun and stun the !#@* out of him", at which point someone else chipped in with "Yeah, I'll do that too!". Given that the guy was bound and on his knees I didn't bother rolling for the shots and just said he collapsed under a volley of stun...my question is; how long will the poor guy be out for?

However long the GM wants it to be...

Cool, that's pretty much what I was going with I just wondered if there was anything specific in the rules how long a stun from a blaster lasts.

For NPCs the rules are pretty set: Minions and Rivals take Strain Damage as Wound Damage, while Nemeses suffer Strain Damage normally and can even recover Strain. But since this scenario you described seemed to be more cinematic in nature, a stunned target probably won't recover until you, as GM, determine it is appropriate to the plot.

Depending on how far the players take it he might even never recover. Stuns can still cause critical damage and I can imagine that if someone is exposed to excessive amounts of it their heart or nervous system could eventually give out. Would have to be up to the GM to make such a ruling though and might not be popular with players.

A couple stun bolts should be fine. Five or six and you might see lasting damage that should get checked out by your local medical droid. In most cases, stun durations tend to be measured in the low range of hours. So, 1-5 or so. But yeah, it's definitely narrative - if it's useful for him to be out for a day, knock him out for a day.

I'd leave him out until someone rolled a despair and have him somehow come back to bite them. Otherwise he's out of the session, for reasons described above.