It depends. Why did your system loose charge in the first place? Probably a very fine leak. The problem is, R134A molecules are smaller then R12's, so a 'small' leak for R12 is a huge leak for R134A.

When you do this, you ideally need to replace all the seals in the system. The best results come this way.

There is a web site somewhere around here that sells a special direct R12 replacement that is still legal... for like $8 a can.