What I've read is 18 months to 3 years for an affair to run it's course. At that point it either fizzles and falls apart, or turns into a "regular relationship", which is to say that the fog of "everything is awesome" has burned off and there are regular relationship problems that come along with the benefits.

Acc

Married 18, Together 20, Now Divorced
M: 48, W: 50, D: 18, S: 16, D: 12
Bomb Dropped (EA, D): 7/13/11
Start Reconcile: 8/15/11
Bomb Dropped (EA, D): 5/1/2014 (Divorced)
In a New Relationship: 3/2015