The last one lasted  about a week.  The previous one about 3 weeks. The one before that about 1.5 months.  I am considering changing toilet to a Gerber or a Church and never buying any American Standard Products again. Since the Champion (sic) tank requires proprietary flush valves, AS could at least make a decent product. Of course the problem could be with the surfaces the gaskets are intended to seal and they were newly installed a few months ago.  The customer service rep said I would be sent a couple  new flush valve gaskets as they were known to have issues.  My research indicates that the gasket issue is very common.