The time to consider the offer of Ada Dormitory is 7 days (subject to the offer). If you decide, you'd better confirm it as soon as possible.
PS: The dormitory service sometimes has an oolong incident. On the third day after I received the offer, it sent me an email informing me that my offer had expired. I wrote to the accommodation department, and he replied that the time on the quotation would prevail. So, if you have any questions, write to them!
As for payment, I also pay monthly. A nominal deduction of 1 pound will be made after "confirmation". Change a local bank card when you arrive in the UK, and fill in the information on MyEd website as required.