All ya need to do is leave your amp on for a day or two. Get yourself a good cheap thermometer and lay it near the hottest part of your amp (heat sinks).
Check the temp and remember. Let amps cool as a normal part of listening.
Check cold temp and then redo checking temp until it gets to above temp.
Now you can turn your amp off and know how long it will take to get to proper operational temperature. Win-win. You're welcome.