Hi guys
Not so much a question about immigration but related anyway
I am currently in US on a visa waiver, valid for 90 days
I'm a UK passport holder and would like to know how long I have to leave the US for, having enetered on this type of visa, before I can return.
I know I cannot just cross the canadian or mexican border and return and I probably only need to do this once as I am hoping I will be applying for Fiancé visa if the lovely lady in my life says yes

Any help would be very much appreciated

You can't simply cross the border to Canada and return in order to get another 90 days. For now, you should leave before you accrue overstay period. There is no exact rule on how long you can come back, but better use some commonsense... if you are in US more than in your home country, it proves your intention.