Overcharging is as bad as undercharging. Few of us home mechanics have the skills and equipment to do well on ac issues.

Generally speaking, an ac charge is forever unless there is a leak. My Dodge truck still works fine after 15 years and so did my 1988 560SL after 25 years. I think you also have a leak and may need a hose replaced.

The shop may try to sell you an R134 conversion. Don't do it unless all of your components (compressor, dryer, etc.) are shot. I had a conversion done on my 1983 300CD for that reason. It works fine but it was expensive.