Hey Gina!
Im actually new here, too. I was usually a guest, until I decided to become a member.
I recommend moving your frogs terrarium someplace else, because in the Winter, being next to the windiwn won't be too pleasant. Crickets are good, and you should try dusting them with vitamins every once or twice a week. You can find that at the pet store.
As for your questions, I think you mean substrate, which is the bedding for your frogs (please correct me if I'm wrong XD). This should be changed every time you clean out your frogs terrarium. Some people do it twice a month, but. I do it once a month. It depends on how many and what size frogs you have.
Frogs are potential carriers of salmonella, but not all of them carry salmonella. That is why it is important to wash your hands before and after. Tip: Before holding your frog, wash your hands with water ONLY. Afterwards, wash with water and soap.
Frogs live can live for quite a while, but it depends on your species of frog. You might want to do a little research on your frog if you got it from the wild. I was given two frogs, caught by a friend. It didn't take long until I found out they were chorus frogs! :P
I hope you have fun with Olive!