Heating - how much time does it take to heat fifth wheel?

There's not really enough data to answer the question. Those owning the same RV can likely answer better but it totally depends on outside temp, inside air temp, AND how long it's been at 32degrees (+coach construction, etc.). If you've left it long enough it's going to take at least an hour to warm up the entire interior (different from just air temp) to the point that the heater doesn't continuously cycle from the entire RV being cold soaked.