I changed the compressor on my car last weekend. I had the line of the compressor disconnected for 1/2 hour. I had to go to a corner garage to charge the system.

Question is how long should the system be purged when it is opened. The guy at the garage did it for 1/2 hour Is this enough, or will there be moisture inside.

Should be plenty of time. I just had mine retroffited with 134a. I replaced the drier, compressor and a few O-rings that were easy to reach. Cleaned out a few lines and condenser. Buttoned it up, put a vaccum on it at the shop for 30 minutes and filled with 134a. Boy is it ever cold now! ('81 380SL)

If he was using a good wet vane vacuum pump, 1/2 hour MAY be ADEQUATE. The problem is that ANY moisture left in the system will combine with the refrigerant to make an acid. The acid of course will cause serious problems eventually.
R134 is worse about this than R12.

If you can pump it down for 24 hours it's best.

Good luck,