Commercial pool filters run 24/7 not residential... residential filters (that are working properly) should be run 8-10 hours daily... i would run it during the hottest part of the day..
You should backwash and rinse weekly...
Chemical usage depends on way too many factors to have a set amount... I HIGHLY recommend getting a salt chlorinator... it will save you so much money in the long run.. it also makes the water more clear and better on the eyes... they're about $1500 installed in the states...
If you were spending $200 a month in chemicals last year but now it's running $300... something is wrong.. doesn't necessarily mean the sand either... it could be your pump.. leaky seal... clogged filters... improper balancing...
I have an idea... kick everyone out of your house and let me stay in it... you can fire all your maintenance/cleaning staff and I'll do it!
thought it was worth a try..