Only you can decide when you think you have had enough.
These cars are not cheap to maintain as you are aware but there is something about it that makes it worth while.
My 96 has 208K miles on it now and I feel that I have gotten my moneys worth but I am still getting everything fixed that goes out.
Just this year I spend $2800 even though the car is probably worth $5000 tops.
But someday I will toss in the towel and dump it, hopefully after another 100K miles.

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2010 W212 E350 Sport
1996 W210 E320 (220K miles)