What temperature would they be at in their natural environment? That is the answer to your question. But, 28C is very hot, unless you are planning to keep tropical species.
Follow the advice from Cris. Or, perhaps 22C is even better. It depends on the species.
28C is fine, I kept turtles of different species at this temp for 2 years without issue. Too low a temp is going to do nothing but cause digestive issues.
Well, actually 28C can cause metabolism problems. But, like I said, it depends on the species. Without a species name, we're simply talking in circles. For example, some species are still active in 4C water temperatures, but I would never recommend keeping your turtles at that temperature.
Some species would rarely experience 28C, so keeping them at that temperature all year round would be naive.