My BT54 front tire was worn down to the wear bars at 7200 miles. There was plenty of tread left on the rear tire. Unfortunately I could find replacement BT54's so I went with BT45 replacements for both front and rear.
I check tire pressures before every ride and run 36 psi for the front and 42 for the rear.
I've got 6700 miles on the BT45's now and the front is starting to cup a bit so I'll probably raise the pressure to 38 to see if it smooths out. The rear tire is flattened on the crown so I guess I've been riding on strait roads too much. Tread depth is fine on both tires.
The BT45's have 4.2/32 inch tread depth on the front and the rear has 5.7/32 inch on the rear according to
this site.