I am sure someone else will give more info.
I too tired to quote the reg, but as far as I understand it, a 121 or 135 pc or checkride resets their commercial cert.
then when you leave the 121 world, then you would be good for 24 calander months since your last PC.

Thats the way I undertstand it, and I am sure someone will be nice and post the reg,

One good place to look would be part 61 for your Cert that you hold. That would be a good start.

Reeves