I'd replace the cables and housings yearly or thereabouts if you're riding the bike all season. Also, having Ultegra equipment doesn't necessarily mean you have Shimano cables & housings... manufacturers will often use Jagwire or whatever.

If you're comfortable with the task of installing a new inner cable, a simple solution to your original question is to go get a new cable now, and you can lube it as desired while you install it. If your housings have sealed end caps, they'll peel off a lot of the lube, so when you're applying lube, pull the end cap up the cable and apply lube "downstream" of it, then push the housing's end cap (commonly called a "ferrule") back onto the housing. Personally I prefer non-sealed, as the cure seems to be worse than the disease, but hey.