What Is The Longest You Should Tan At The Beach In 1 Day?

For vitamin D absorption you don't need much time and without sunblock I would not spend more than 30 minutes and WITH sunblock I would not spend more than 3 hours. When I was younger I would tan all day but now I am trying to take care of my skin for when I am really old. Just the reality now.

It is impossible to say, it depends on the strength of the sun, whether you are already tanned and the type of skin you have and whether you use a tanning protection product.
In full sun without a tan and red hair/pale skin, 30mins max, dark hair and less sensitive skin 3 hours.
Me, tanned already, dark hair and skin, all day no problem.
If you have sensitive skin, it is sensible to expose yourself to the sun for short periods, often, and gradually build up a tan, then you can safely spend a lot more time in the sun.