I think "Family Hour" is the longest one I know of but that's most definitely an alt-universe.
The longest time for a pretty much cannon universe has Lois figuring it out about 1 year into the marriage. She tries to get Clark to open up, she even arranges to get him all jealous and confused by seducing Superman again.
Grr I can't find it, though. Can't remember what it's called.
