When you have your hot tubs temperature at 100 F degrees you should not stay in the hot tub any longer than 15 minutes at a time especially if you start feeling faint or dizzy.
If not then you can stay in the hot tub a bit longer but I would not recommend going any longer than 30 minutes in a hot tub of 100 F degree water or any temperature as you can get heat stroke as a result and possibly die as well.
When in a hot tub try to take a break every 10 to 15 minutes and then stay out of the hot tub for at least 15 minutes before returning to the hot tub.
The maximum length of time that it's safe to stay in a hot tub at one time without a break is 10 to 15 minutes.
When using a hot tub it's best to get out of the hot tub after 10 minutes to take a break and cool off to prevent getting overheated.
When you stay in a hot tub too long your core body temperature increases and can increase to dangerous enough levels to cause heat related illnesses and even possible death.
Some people have stayed in a hot tub for 30 minutes without a break and they've been okay but if you start feeling dizzy, faint or just getting too hot before then you should get out of the hot tub.
When I use my hot tub I can not go longer than 15 minutes before I get too hot and need a break and then I take a break from the hot tub for at least another 15 minutes before returning to the hot tub.
Hot Tubs are safe when used correctly and you don't stay in it too long but if you start feeling bad and getting too hot then get out of the hot tub before you get too ill.