The guarantee as printed is that the product will last ten years without showing growth of black mould. Less than two years ago I did my shower tray with it but it got black and I have had to do it again already. Couldn't use the remainder of the (nearly full) Foreverwhite dispenser for two reasons, firstly because thereon it stated that it can't be used one year after opening and secondly, under the terms of the guarantee Printed on that container I was required to return either the original receipt OR the container. Not having the receipt I returned the container and asked for a refund (why not, they charge over the odds for the supposed benefits). After waiting a month without a response I wrote again. In both letters I confirmed that I had complied with their requirement to regularly clean the surface to remove soap build-up as is a requirement for a ten year stain free job. This second letter prompted a reply in which they claimed they hadn't received the returned item - for which I had obtained a certificate of posting which I did enclose in my second letter - and, worse, they said their guarantee required the production of the original sales receipt, obviously ignoring the other option within the terms of their own guarantee.
Happily they sent me a refund cheque at this stage but only under protest viz: a paragraph in an ungracious letter which stated "with this without prejudice payment (we) now consider this matter closed".
So, if you buy it keep the receipt. When it goes black send it back.
Incidentally they also referred to the fact that they had been unable to do any " in house testing of of the removed sealant with the reported mould discolouration" so include some with your complaint should you be as unlucky as me to get it.