You just pick one firing time. You do have a choice though. You don't
say which version of PMC you're using. If it is PMC+ your choices
1470 for 30 minutes
1650 for two hours
and there's a lower temperature, but I don't remember it, because I
never use it.
It's one or the other.
Check your clay package, there's a little piece of paper in there
that lists your choices.
Metalsmith, Certified PMC Instructor
Hard to Find Tools for Metal Clay