A slip could be chain and cassette wear mismatch or slightly out RD adjustment.

You could check on the 2nd or 3rd largest rear cog and make sure the chain is dead center on that cog. You should see equal slivers of light on each side of the chain looking straight on from the back.

Depends on the weather, but you should get 2000 on a chain, 4000 on a cassette and 8000-10000 miles on a chain ring. That's more of a rule of thumb though and a lot of YMMV. It is also assuming you keep your drive chain clean and lubricated and that you don't spend huge amounts of time in favorite gears.