Partner's pretty much on the money here. It's not key cycles that are
necessary, but the completion of the evaporative self-test with good
results. The reason it took so long for the lamp to come on is that the
test had to run and fail twice in a row in order to turn on the lamp.
Now, it must run and pass three times in a row to turn the lamp out. And
then, the lamp will go out, but the code will continue to be retained for
quite some time.
Considering it's a pretty safe bet that the gas cap was the issue, I'd
that you can go ahead and pull the negative battery cable off for five
seconds if you desire. This will not only turn out the lamp but clear
code as well. The only drawback is that if the fuel cap wasn't the
that you've just erased any capability a shop would have had of
determining what actually occurred to set the lamp.