We had 100+ days here in NC this summer. Cruiser goes out to do his business and usually demands to go back in immediately. In the mornings while its cool he stays out for about an hour. Now that the days are getting cooler 90's now (looking forward to real cool weather hopefully soon) he still has to be pushed out and will play but drinks like crazy.He has a kiddie pool also, all of my dogs have had one. It took him a bit to get used to it and he loves it. Cruiser goes to day care and they go out for play, he does pretty well as long as he gets a good amount of water. All mastiffs are heat sensitive, but tolerate it better then they let on. All four of mine complained but did ok. Your girl will be just fine and will be so distracted with social time she won't notice the heat until you arrive, of course she will make you think she is dying, they are pretty dramatic! LOL