How long should it take to a 1.5 tons AC to cool down a 2000sqf house from 80F to 77F?
As written this question is pretty hard to answer.  It sounds like you're having AC problems tho and need a test method.

Find or purchase 2 probe type thermometers like thermometers. Analog or digital doesn't matter, the only things that are important are that they have a probe and that they give the same reading laying next to each other on a table (you will be surprised how many don't).  You can purchase these many places for under $10, there normally used in the kitchen.


Take these to the area of your furnace and find someplace to stick each in, one in the return air, one in the supply air.  There is always a crack or hole in the ductwork somewhere, I have yet to see a case where a hole needed to be made.  On the return air side, the simple place is where the air filter is located. The supply side is harder to find sometimes, but just get the probe into the airflow.

Once the system has been running 15-20 minutes the thermometer should be reading temps about 15-20 degree's difference.  



If your aren't getting 15-20 degrees of cooling you may need a service man, but check the filters first, and check airflow on the outside coils as well.  They typically need to be hosed off once or twice a year.