Typically every 3 is a good plan. You should still be in the top raking off the sand bed and checking the level of sand every 6 months or more if you have a lot of trees or dogs.

this is our 3rd year with a pool and the first year changing the sand.... now that we have did it sucessfully, it wasnt bad..... but OMG! doing it was NOT FUN!
we had a bad bad bad algea problem and clearing it up required us changing the sand...(it worked)
first we closed the valves off and unplugged everything
then we loosened the hoses and all that
then we removed the collar on the filter we then removed the head, but was left with the inside shaft and fingers in the bottom
then we shop vac'd the sand and water out, and adding more water to the sand as it would become quite solid
we did that until DH called a friend and found out how that we can unscrew the fingers at the bottom (DUH!) and remove the shaft (DUH AGAIN!)
then we simply turned it bottoms up and sprayed it out. very simple once we learned those tricks.
when clean, we put a little water in the bottom, reinstalled the fingers on the shaft, and placed a long cup over the shaft top and slowly poured in the sand. it worked like a charm ,,,
you will hve to check with your brand of filter, to see if its similar.
hope this helps.
zz