It depends, some people for offerings burn food, or things in offer for the gods. Other's just bury there offerings once a month. Either or, offerings I would stick to two offering bowls, one is for food, and natural resources, this bowl can then be burried. The either bowl can be say your things you can bury, then you could always out it back, and charge it again. Good luck!
They aren't banished. They stay we're they are. I would change them out every once and a while. It's good to give offerings to deities. If it's a offering to a fore deity like surt for example I think the offerings should be burned. Or you can give them a offering like coal or ash for example. As I said it depends on the deity. Hope this helps!