Attempting to compare the length of Mahut and Isner's marathon tennis match to other sports. I'll be making some questionable assumptions*, but they are fair enough, and necessary for comparing two different sports.
Mahut and Isner's 68-70 fifth set match is akin to...
... a college basketball game going to 20 overtimes. (7 overtimes is the record)
... a major league baseball game going to 32 innings. (25 innings is the record)
... a NFL game going to 10 overtimes. (2 overtimes is the record)
... a golf match going to 45 extra holes. (11 extra holes is the record)
*Assuming an average tennis set has 10 games (6-4 sounds average-ish), and the ratio of the games played in the first five sets (50) to the remaining extra games played (128) means that the overtime is basically 2.5 times the regular time.