it depends on the type of tank you want. fish only (FO), fish only with live rock (FOWLR), or a reef.
you can have a SW tank with no LR at all, but your filtration will need to take up the slack.
the general rule is 1-2 lbs per gallon, so 1500 lbs is a good place to start. but the volume of 1500 lbs of LR will vary, as the lighter more porus (better) rock will obviously weigh less than a less porus piece of the same size.
if you want to use less LR, get some base rock as well. a 60/40 mix of live to base is good. its still 600 lbs of LR, but its gonna be a hell of a lot cheaper than buying 1000 lbs.
or just go FO and just use some massive filtration, and also massive skimmer(s) and waterchanges