Toilet P trap has water in it to keep the fumes from coming back into the house. When one goes on an extend vacation like 3 months or longer. I assume the water in the P trap will evaporate. I am looking for that time frame?

Pour a very thin layer of cooking oil on the water, it won't evaporate.

Brian gave you one solution. However, in Florida we have winter visitors that leave their condominiums for six months out of the year with no problems. If you are we concerned then pour cooking oil in every trap in the house. Have a great vacation, Tom