What is the deciding factor in how long before you change?

I'm not sure of my res. temp because I don't have a thermometer for the rez (only a digital in/out that had the "out" sensor tore up when it hit a fan blade). With all the reading I'd done before hand, I didn't see much talking about rez temps, but in my posts people have asked again and again. What is the proper temp? Mine is cool, guessing between 70-80 degrees based on external temperature.

I have to wait until my apartments fix my A/C, but since the apartment temp right now is just under 80, I think when it's fixed it should be able to bring the temp at least to between 70-80 (i hope). During the day I have one of those tower fans blowing air into the closet from the room so it has cooler air to circulate.

I usually keep my tent zipper open half way since it creates such a vacuum and I don't want to burn the fan motor out. Am i being overly cautious or is that a real issue?