John,

You can use Weather.com to get some idea about temperatures at different parts of the park, including Panther Junction. The Big Bend website has some good info on the variation in temps and some links for more current data for higher areas (Chisos Basin) and lower (Rio Grande Village). One has to be careful here since things obviously can vary widely and temperatures on the same day can vary widely depending upon your elevation. The elevation along the South Rim should provide cooler temperatures than those found at the same time a few thousand feet below.

As far as time and distance goes, you should be able to do a loop that includes the South Rim from the Chisos Basin in a single day. You mentioned 7-8 hours of hiking and that should be doable. Check out our hike along the south Rim in 2001. Yes, the mileage and time exceeds what you might have in mind, but note that we foolishly did the South Rim loop and hike to the top of Emory Peak on top of that. That was too much.

Even if pushing it for a single day is too much you can easily do the entirety of the South Rim loop in two days with a single overnight at one the camp sites along the Rim. Actually, given the views there, camping along the Rim would be highly recommended. I do not believe camping outside of camp sites is allowed in the area, but you'd have to check with the park about that.

Bring all the water you'd need is my advice. Though there are streams in the mountains, none of them are particularly reliable as I recall. If you're talking about a day or two, we're not really talking about too much weight.

Good luck!