What's the normal psi range for sand filter? Just bought a new pentair tagelus 24" sand filter but I don't know if the psi is good or not....I know if it goes up 10psi then it needs to be backwashed but how do I know that my normal level is acceptable. It has a max psi of 50.

We reused the sand from the old one as it was just from last year I believe. How do we know when to replace the sand?

Your normal pressure depends on your pump, plumbing design, etc. Every pool is different. What is your pressure now compared with your old filter, and what type was the old filter?

We just bought the house and we're new to swimming pool maintenance so we have no idea. Max pressure for that filter is 50 and it's currently 12-15...just wanted to make sure that's not too low and was normal.

If it's running 12 to 15 now that's fine. If you do some reading you'll see that mose sand filters here run from 10 to 18 psi.
Keep an eye on it and when it rises about 6 psi or so, backwash it and see what the pressure returns to.