it depends which office you go to, in krabi they require 3 months I was 5 days short because I had a bank fee come out a few times, there's also a multiple entry visa which you renew every 3 months but not sure if its doable with all the restrictions, some say 2 but do 3 just in case
this is incorrect Information! I have just done it. It doesn't matter where the money comes from as long as you have 400k in your account for 3 months without withdrawals alll is OK but it does have to be in a thai account it doesn't matter where the money is from as long as its in a thai account you do not have to do it gradually I had my wedding in February diposited all the 400k in at once and there was no problem as I had it for 3 months
You can extend in country if you have a multiple visa already. You need 400k in your account for 3momths pictures of the wedding, of your house and just from random times together you also need a map from the immigration office to your house