A/c is squealing have gone through two belts. How tight do i tighten the ac belt

As far as the belt tension goes - without a guage .. About 1/2 inch deflection when pushed (comfortably) with thumb at the longest belt span .. While listening carefully in a quiet environment, belt should be tight enough to vibrate and make a low note tone if strummed (push down with thumb and quickly slide thumb off belt) like a string instrument.