Maybe others can chime in, but in my experience the seals seem to last forever, as long as the pads aren't worn too far down that the pistons start protruding farther than they normally should. When they come out too far, often the seals get stressed to the breaking point and/or don't retract properly when the pads are replaced. I have had calipers start leaking on me under these circumstances.

Just my $.02.

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280