Yeah in full suit I have about a limit of 1 maybe 2 hours before it gets too heavy. The heat isn't so bad it is more of the weight, especially if I have to do a lot of posing with the arms up in position. The upper body weighs 55lbs with each arm about 15lbs (overall weight is 75+lbs if you include the stilts). Most of the weight on the arms are the hands too which makes the arms feel heavier. I was walking around in just the upper body for a while because I can travel from one spot to the next faster that way and my handler pulls the top via dolly behind me.
If it was on Friday when you ran into me, then yeah I made the mistake to walk the whole costume in. I learned big time from that, I also had one of the legs of the costume loose and had to struggle with that all day. Even in half suit I can only go so long without having to stop, my stilts are adjusted for standing still, the springs are pretty tight and walking is tough. Though I have been known to "run" in the stilts, I did have to do it once to catch up from the folks at the Droid room.
I fit pretty easily into an elevator, the folks with me wouldn't let me do the escalators, though I think I could do it!