It is correct that the bank can take as little time as a day, or as much time a few weeks to returned a fully executed contract. It all depends on a number of factors, one of them being staff availability and the number of files they are currently processing.

It is not a fully accepted agreement until they return a signed copy. I have seen it take 3 weeks to get a signed copy from a bank but have also seen them returned in a matter of hours. There is no rule requiring a specific time unless one was specified in the contract.