How long could you keep a medium sized, brown, healthy, 2 years old dog outside in full sun without water on a 95 °F (35 °C) day without risking sunburn/heatstroke/etc.?
Clarify from comment posted by OP: a dog was kept in that situation for an hour and a half, got into a debate about whether that was too long and went looking for sources for backup but none give a time limit. Argument was "well you take a dog to the beach, right?"
Dogs are similar to humans on a lot of aspects. If you cannot stand it, neither can they. If someone was trying to do that for instance just to see, something is wrong with them, call the local authorities. Beagles are used to do a lot of horrible things to them to see if it is safe for humans and it makes me feel bad to think that make up I wore was tested in a beagles eyes just to see if it hurts, so that we don't do it. Don't ever take a dog and practice or do anything just to see what happens. A lot of people forget when they walk their dog that the street burns their feet too just like it would burn you. Please be careful when walking your dog. The street is hot on their paws, too.
Another way of looking for a solution to your question is this.
The question asked a different way...
Of its own choice, how long would a medium sized, brown, healthy 2 year old dog remain outside in full sun without water on a 95 degree day?
I suspect the answer would be a very short period of time. The dog would naturally seek shade and water.
