Your filter manual will note when the pressure is high enough to require backwashing. The one I found online (that may not be your model at all) says normal running is below 8 lbs, and backwash when it get to 9 lbs. But find your manual, and see what it says.
No harm, if you are conservative and backwash at a lower pressure increase. Having slightly dirty sand increases the ability to trap things (and a bit of pressure increase), but should be paid attention to, since by trapping things more the pressure will rise faster.
If you will be gone, and leave it running, there is a chance it will catch too much, and nobody around to backwash it. Likely will not happen, but you'd hate to burn out the pump. Me? I'd just turn it off. Buy a floating Cl dispenser - pool store, most big box stores (Walmart, etc.) and put a couple of your tablets in that while gone. It will be a guess as to how much Cl it adds, but will help to some extent.
The downside to turning it off, you'll likely find that there will be piles of dead algae on the bottom of the pool on your return, needing brushing or vacuuming (if you have one) to get it out. So the unknown in this is what ability/tools you have to get dirt, etc. off the bottom of the pool.....