When I arrived on my fiancee visa, I had written "indefinite" on my landing card, as even though the visa is good for 6 months, the length of my stay will be indefinite as I don't have to leave to apply for FLR, and I do indeed plan to stay in the UK indefinitely. The IO kind of crabbed at me that I should've put 6 months, as that's when my visa expires, but she let me go on without having to write another one.