How long can one stay in a hotspring ?

I would like to find out how long can one stay in a hotspring/ bath ? Would it pose a danger if stay too long.

Yes, it is dangerous to stay in the bath too long, or take too many baths per day.

The details depend on the temperature of the bath. If the water is hot, it is better not to stay in the water for more than 10-15 minutes at a time, spend some time with your upper body out of the water and take breaks in between.

Then again, it is is a luke warm bath (38 degrees or less), then you can spend longer time in the bath. But most onsen are above 40 degrees.

Much like hottubs, each persons personal resistance level to the heat will be different. In these types of cases, it is usually suggested that you leave the hot water if you begin to feel sick, light headed or dizzy.

If you have a private onsen, or you are in your private bath at home, a good way to be able to prolong your time in the water would be to place a cold, wet flannel or towel across the top of your head or forehead. Taking sips of cold water may also help.

Just don't push your limits too much though. The last thing you want to do is faint while you are in the water.

How long you can stay in depends on your physical condition & your tolerance for heat. I can take about 5 minutes for full submersion & then gradually raise up until just my legs are in. After about 20 minutes, I have had enough. My wife on the other hand, can stay in for a very long time, sometimes 45 minutes. I don't know how she does it.
If you have heart trouble, high blood pressure, are overweight, have diabetes or a couple of other medical conditions, you should consult your doctor and be very careful.