I've got a P210 converted to R134, it takes a little longer to get cold, doesn't get as cold as R12, however it still gets cool enough in the cabin on those hot summer southern California days.
It usually takes about 2.5 cans and it's topped off. Check to see if you have a hi and low pressure switch in the system this will help also when filling not to overfill and safeguards the system. Lance F is correct the pressures should be 25-65 on the low side, 40-45 is ideal, 100-275 on the high side ideally about 170-180, I have both high and low switches installed and so far so good no problems fill it till I don't see the freon flow in the gauge manifold window. Pressures reach 45 on the low side and around 190 on the high side if I remember correctly from the last time I charged the AC. If you do convert to R134, you'll have to change the fittings to 134 fittings (R12 fittings are becoming more and more rare on manifold gauges), make sure to vacuum out the system and purge any R12 that may still remain in the system, add oil, check for leaks and fill in R134.