Please help me with this question if you can

I am a UK citizen currently living with my fiance in the USA. We plan to get married next summer in the UK and only intend to stay in the UK for about a month and head back to the USA after we get married. My understanding is that he will still need a 6month visiting visa right? Which is about 75 pounds..
I am unsure as to when we should apply for this. Nothing should get denied but I want to make sure that we have the visa before paying for wedding stuff in the off chance that something could go wrong. But how long can you have your visa without it expiring? If we sent off for his visa in December and didnt go to UK to get married till mid-june would this be too long of a wait? Also, what are the processing times on average for this type of visa?
Thanks!!