I know in d20 games it's generally considered to be 6 seconds but does anyone know how long it's supposed to last in the 40k rpgs?

Well according to P187 of ye olde Dark Heresy Rulebook:

"A round is approximately five seconds of Action".

Deathwatch (P234) agrees with this:

"...so a Round is approximately five seconds long."

So everyone does all their moves, no matter how many and such within a five second slice.

That's good to know. I thought I had read it somewhere, but I couldn't find it in the BC book.