I'm currently working on a fantasy novel and I've reached a scene where a character wearing full harness has been immobilized in the sun. Given a 90+ degree day, how long would it take for heat exhaustion to set in? The character in question is in his late 30's and in relatively good health, just a bit out of shape given a relatively leisurely lifestyle. He was drinking before the incident occurred, though not heavily. I only need a rough estimate. Thanks in advance.

Aldian,
Though not a direct answer to your question, here is a previous thread that talks about armour and heat: http://www.myArmoury.com/talk/viewtopic.php?t=19147 .

If he takes his helmet off regularly then he won't suffer any more than if he was wearing heavy clothing.