One minute as a base, plus Co mod x2 in rounds. For every five ranks in swimming, add 1minute.
When the limit is passed, a Sd check is made. If successful, another check doesn't have to be made till Co mod in rounds pass.
So, 1 minute + Co mod x2 + 1 minute/5 ranks in swimming for free (unless struck, grappling, surprised, etc).
Hard Sd check is made, may hold breath as many rounds as Co mod, then:
VH Sd check, if made as before.
Ex. Hard check, etc, etc.
As soon as Sd check is failed, N/PC drowns.
P.S. Use a paper clip on the difficulty column to remember where you are at, particularly in melee. The next check may be a half hour away in real time if in tactical mode.