In a residential pool, you don't ever need to change the sand unless you've used bacquacil or some other additive that would gunk it up, although you may have to occasionally top it off due to sand lost during backwashing. In a city pool I can't think of a reason why that advice would differ, but the only person around this forum that has experience with large commercial pools is Pooldoc, so he would be the one to answer that question definitively. He's been away from the forum for a couple of weeks, but hopefully will be back soon.
In the meantime, you can log out, go to the main forum page, and then find any post made by me, Watermom, or Pooldoc. In our sigs is a link to the google search feature, which will allow you to search the entire forum and archives for other posts that may give relevant information.
Edit: when new users post, their posts to into a moderator queue and wait for one of us or Pooldoc to approve them before they make it to the forum--it's a step Ben put in place to keep the spammers and hackers off the forum, and it's why you didn't see your post appear immediately. I have deleted the other duplicate post in order to keep all the views and responses in one place.
Welcome to the forum!
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