A few minutes would cook their brain in that situation.If it is 90 degrees, on concrete and in direct sun? And the tort was already pretty cool from being inside? Sorry for such a vague question.
I would never put a tortoise in direct sun with no shade. You need an outdoor enclosure that offers sun AND shade, and the shade needs to be from high over head like a tree, umbrella or awning, not from a board laid on top of the tub. We say "sun" out tortoises, but what that really means if you explain it in more detail is: "Put them in a safe secure outdoor enclosure that offers the correct amount of shade and sunny areas and the correct temperures depending on the time of year. More shade in summer, less shade in winter. Always check the temperatures early and often." We are offering access to sun, not full sun with no way to escape it.