The Service Engine Soon light came on the other day, so I checked the owner's manual which said it's an emission control factor, but suggested first making sure the gas cap was on properly.

It went on to say that - if it was the gas cap - it could take several trips before it went off. Emailed my favorite service advisor at the local Infiniti dealer, and she said it could take several days to go out.

Finally the question: if it is the gas cap, why does it take so long for the SES light to reset itself and go out??

 

Did your ever actually check to see if the gas cap is properly tightened?
If you never actually checked it and it is not tight the light will not go out

Telcoman

 

I checked it immediately. It's been going on two days now, and the light is still
showing.

 

You need to scan it and find out what code is throwing and then research what could be going on

 

I think you're right, but the dealer's going to have to do it - I'm completely clueless on stuff like that.

 

Autozone etc can scan it for free

 

I second the AutoZone scan. I work there and do it many times a day. They have a system that has been spot on many times. Oh, and its FREE! They can even give you a print out so you can research the issue on your own.