When I went to the UK last year, I ended up staying for roughly 4 months total between two trips. Looking back now, I was extremely lucky to get through immigration, but I know other people who have stayed over in the UK for similar amounts of time. It all just depends on how strong your ties to the US are, since your biggest hurdle is proving to them that you're not going to overstay your visit--things like employment contracts, rental agreements/mortgages, schooling, etc can help. I'm not sure how well employment would play if someone were to stay in the UK for three months (unless you had a sabbatical or something to use), but it depends on your particular situation. Generally though, the stronger your ties to the US, the better, and being honest with the IO is a *must*.
As for the marriage thing--it actually doesn't take long to do once you decide to do it for reals. My husband proposed to me last November, we were married this last May, and it took me a month from the time he left to get my bios done and my visa approved (and I only took that long since I was waiting on some paperwork, it can be done in less than that if you have everything together right after the marriage). But, IMHO, it has to be something that you and your BF would want to do irregardless of immigration--of course it's always in the back of your mind, but you have to make sure it's not just a means to an end. Once you get the visa, you still have to be married to each other, so it should be a natural progression of the relationship and not just a way to live together. So, it may take some soul searching on your part, and maybe a long talk with your BF to see what your goals are, and to see if marriage is a viable option at some point down the road.
But it is a really difficult process to go through at times, and the worst seems to be right before one person leaves
Some of the worst points of my life were those tearful goodbyes at the airport, and at times it seems like there's no real end to the journey, like it'll be just a bunch of trips back and forth indefinitely. With that said, these stories can have happy endings, it just requires a lot of work, and thinking things through, and a lot of soul searching to see exactly what you want. But it can be done, it just depends on what you two want as a couple, and what you're willing to do to get there. It's definitely possible though