I've had a lot of differing answers to this, hopefully you guys can clear things up for me.

I need to know exactly what is required to help my (unmarried) partner visit the UK from the US. Just a holiday. We don't know for how long, but basically we hope for it to be as long as we can afford.

We just want to know exactly what needs to be done. She hasn't left the US before, so we're applying for a Passport which seems to be relatively straight forward. However, and I've heard this a lot, the visitor's visa is a bit confusing. Does she need one at all? If so, how would it happen and what would be required for it?

The main part we're having problems understanding is about how much is required to sustain oneself during their visit. There doesn't seem to be any relation to how long you stay and how much you're required to prove you have to sustain it. Is there some sort of measure?

And, since she'll be staying with me and my family, is there any way we can prove she'll be cared for by us and thus could rely less on her own funds?

Any information will be greatly appreciated. Thank you.