That's a really fun activity you have for kitty there. Just be alert to his habits and make sure he doesn't start acting different. Stress in cats can cause a multitude of problems, and cats tend to not show their stress very outright so just pay special attention to how well he keeps going to the litter box, as many problems will first manifest there. I'm not saying your kitty is stressed by this, actually he sounds rather the opposite, like he thoroughly enjoys his visits, but cats are very good at hiding their stress, and can be stressed very easily, so it's better to keep an eye out for it!
Now another note, socializing cats isn't necessarily the same as socializing dogs, though I'd say this is a good way to get him used to other cats, it's no guarantee that's how he'll act in his home environment. As you can already see, a lot depends on the particular cats he interacts with, and the fact that he's in "their" home might make him more friendly, and less confrontational.
But yeah, the amount of time won't matter I don't think. Cats either get along (or tolerate) other cats or they simply won't. No amount of socialization will make them more social towards other cats (unless it is done at a young age). You seem to have a cat that tolerates other cats fairly well in a strange environment, but like I said, you might see a complete 180 when these other cats are encroaching on his territory. No matter how social the cat, always take the introductions of new cats slowly to avoid huge conflicts.
Just an example, the boy we adopted from the shelter was 100% ok with other cats, he was kept in the "cat room" of a local pet store who holds cats for our shelter. Now, I am almost certain if we were to bring a "friend" home he would be very territorial and potentially aggressive towards that other cat, no matter how "used" to other cats he is. He's already proven this when he gets out and the local grey tabby is roaming around our yard. He chases that cat away, screeching all the while. I'm sure with proper introductions he'd be fine, but just bringing another cat in would be a big no no.