I have a house that I have been trapping skunks. get over there and see two dig unders were the skunks are going into the crawl space. set two traps at each hole and fenced off. first night I had two. one at each hole. the next two nights nothing then one on the fourth night. 3 skunks so far. two females and one male. after a week of trapping I crawled under the house as far as I could (very short crawl space) and found a borrow that is about 5 inches across. around the hole the is a lot of skunk poop. I have not caught anything in a week and a half now but when I go under the house there is fresh poop and a pee mark close to the hole about every other day. I would set a trap over the hole but I can not get anything bigger then a squirrel trap in there. crawl space is very low. so low that I can shine my light on the hole but I would have to dig dirt out of the way to get to the hole.
question is how long can they lay up under ground when its hot? talking 95-100 degrees every day here lately.

As far as temps go, the temperature under the house and down in a den will probably remain almost constantly cool. That leaves the outside temp. The nightime temps will be the important factor. If it gets down into the seventies at night, I think the skunks would have no problem with that.

I think any skunk would want to eat and drink each day, especially lactating females. If there are fresh feces and urine at the site, I'd say they are active.

Skunks do use a circuit of several dens though. If you are seeing inactivity at the job site den, they are probable just making the rounds rather than laying low.