It takes about 30 minutes at most. Very easy to do. Should cost you less than $50. You just need to put some new oil into the system and add some adapters that screw on over the old filling nipples.
The hardest part is that it is illegal to release FREON into the atmosphere. You need to take it to someone who does A/C to have them remove it all before you start. Mine was already empty when I did mine. You can always unhook your line to see if yours is empty, too. I'm sure it probably is.
Pep Boys is exactly where I got my kit.
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351W 89 Mustang GT Convertible