How long does it take for underwear to dry in the summer of 28 to 29 degrees?

It depends on ventilation and air humidity! Temperature is not the most important thing.

If the air circulates quickly and the air is dry, it will dry in an hour ~ ~

The temperature in the sauna room is very high, but the air is not circulating and the humidity is high. Dry clothes will get wet if they stay in it for a while.

In that case, three or four hours. In the case of completely natural drying, it is best to expose it to the sun. If you want to shorten the time, you can use an iron. If it's faster, you can use a hair dryer, but it's farther away.