Overuse in horses?

How can you tell if a horse is being overworked? I lease my horse out to a girl that doesn't work him very hard ( she jumps about starter and very low BN) and just rides recreationally. She rides him 3-4 times a week and I ride 4 days a week. I also let my stable owner use him in lessons.The stable owner is in dire need of a few other school horses as she is using her other horses 3 times on her lesson days...which is pretty much almost the whole week. She's also been getting more and more students and using the horse I lease more in lessons, so with me and his owner he gets ridden twice a day a few days out of the week (3-4) this has always been a BIG pet peeve of mine as I don't think its fair for a horse to be ridden twice a day by different people. The weather has been especially hot latley as we live in California (100+) temps and she hasn't canceled lessons either...He is 15 and a half ish I think, still in excellent shape. I event him about Beginner Novice but he can go way bigger. I just want him to be happy and stay sound into older age. The lessons aren't always beginning either, some of them have jumping. Please reply!!!!!!!

Welcome to the forum YOU could tell if a horse is over worked by two ways one-- is the horse losing weight? Two-- is the horse falling over at the end of the day? These could be signs that the horse is over worked there are other sign but these are the ones I look for first

If he is getting sore, soured or losing condition - you know he's doing too much. Just keep an eye on him, he'll let you know if it's too much.

Bare in mind that if he's only teaching beginners lessons, trotting around the track for 40minutes isn't going to wear him out terribly much.

From your post you sound like a reasonable person. It also sounds like your gut is telling you to be a little concerned. Every horse is different and what may be not problem at all for some may indeed be too much for another. This is your horse and if you have a nagging feeling that something may be not be quite right you owe it to yourself and above all to your horse to figure out what that something is.
Have you noticed something different, off, about him that you maybe just haven't been able to pinpoint?
There's a reason you're asking this question in the first place...

There doesn't seem to be much difference except he seems to have gotten a little lazier iin the past week or so. I may just be overprotective but is it fair for a horse to be ridden twice in one day by different people
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They aren't all W/T most are W/T/C and stadium and xc jumping. Nothing to big usually, but still..
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Thousands of school horses around the world get worked often more than twice a day most days a week. They just get fitter and cope with it.

It doesn't sound like he's in trouble of getting worked to death and the heat could easily explain why he's acting a bit "lazy." Just remember though, he's your horse, you know him best and you should never subjugate your best judgement for anyone elses...especially on a forum, no matter how well meaning we are.