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The Olympus OM-2N from the 1970s went to 2 minutes. (I might add, in about half the size and weight of a Canon 5D.) AFAIK that is the longest of any production camera ever made, and it was done, in Av only, by measuring light reflected off the film during exposure.
My Nikon FE2 will happily expose for 15-20 minutes (or more) in Av mode, and it does it quite accurately, too (assuming you correct for reciprocity).