I believe that there's a general rule that I've heard from people on these boards that you should spend as much time outside of the US (or the UK) as you spent in it before returning again. With that said, I first visited the UK in March and stayed for 6 weeks, then I left, then I returned in October for 12 weeks and got through alright (your mileage may vary though, I see nowadays that I got lucky).
But, just as a warning--sometimes IOs get suspicious if you plan on staying for the full 90 days, or 6 months in the respective countries. I guess if you both have really strong ties to your respective home countries they might let you stay for those full periods, but there's a decent chance that they might get suspicious since it is a considerable amount of time to spend on a holiday, even if you're visiting your SO. I can't say for 100% certainty that you two will be turned away, but it's something to consider.
So your best bet is to do a couple of visits back and forth, and if you two want to pursue a spousal visa route, you'll have to come to the US to marry, then she will apply from the US to get the actual visa.
But as for your actual question about what sort of time limit there is between visits, I would give it a few months in between each--like I said, the same amount of time out of the country as you spent in it. If there is a hard and fast rule, I'm sure someone will point it out though.
