School of Hard Knocks (under Swimming) says you can hold your breath for a number of rounds equal to your constitution stat divided by 10. So that would be the base.
Talent Law (and RMFRP Character Law) has the Strong Lungs talent which lets you hold your breath for a base of 10 seconds (1 round) "plus 1 minute per Co bonus (minimum of 10 seconds)". If your Co bonus is +3, that would mean you can hold your breath for the base 10 seconds plus 3 additional minutes. For someone with a Co bonus of +0 or less, they would only hold their breath for the base 10 seconds.
There is a funny crossover here because a stat of 50 with a bonus of +0 with Strong Lungs limits you to 10 seconds, but the rule for everyone in SoHK would let you hold your breath for 5 rounds = 50 seconds, so you are worse off with the talent.... Strong Lungs is only advantageous if you have a Co bonus. That's probably because the two rules were written without any knowledge of each other, but I think it's fair to assume that no one should take Strong Lungs unless they have a good Constitution.