I found that the seat-belt webbing was too soft and flexible for the job and, while it has excellent tensile strength, it tended to fray very easily and didn't last very long.
Some folks aren't comfortable with the stiffness of weight-belt webbing but I've found that it softens up with a little use (toss it in the washing machine for a couple of runs) and that the extra stiffness makes it easier to get in and out of my rig. In my experience, it lasts longer than any of the alternatives I've tried.
There is an intermediate material - call it weight-belt material lite - that is close to ideal but sometimes hard to find. Halcyon and most manufacturers uses it for their webbing, you used to be able to buy this stuff online from Northeast Scuba, I don't know if they still carry it.
I've found that I need to replace my webbing every couple of years pretty much no matter what material I use and I've found that the stiffer stuff doesn't bother my shoulders while making it easier to don/doff the rig. It's readily available and cheap, so I've kind of stopped searching for the grail of webbing. It's easy to use a soldering iron to burn a hole instead of buying it pre-punched, which means I can spool off as much as I need and not have to worry about guessing the exact length. Every body is different so guessing exactly how much you'll need is hard to do. At the long end, a big person will need less than 15 feet, smaller folks might make do with as little as 10.
Oh, I do use the kinder, gentler seat-belt material for the crotch strap...