I would have thought once a week would be more than enough, monthly would do.
It doesn't matter if cable gets wet as long as it is undamaged and the terminations at the ends are indoors. But outdoor electrical stuff is more easily damaged.
BTW if you had the transformer inside, and ran a 12v cable to the car, you wouldn't have this worry. My bike transformer comes with terminals that you can attach to the battery, and a plug-on lead from the charger. I presume you are talking about an old car that isn't often used.
It doesn't matter if cable gets wet as long as it is undamaged and the terminations at the ends are indoors. But outdoor electrical stuff is more easily damaged.
BTW if you had the transformer inside, and ran a 12v cable to the car, you wouldn't have this worry. My bike transformer comes with terminals that you can attach to the battery, and a plug-on lead from the charger. I presume you are talking about an old car that isn't often used.