I seem to have this same problem in my 1990 300D 2.5. A charge will last about a month. The last guy said that his sniffer indicated a minuscule leak under the dash.
In addition, when the system is working, from time to time it will blow hot air for about 30 seconds, then begins cooling again.
A question: what is an estimate for the average job to repair this? How many days would it take?