Let's say you have some cash in the bank and want to chase higher interest rates. While your money is being transferred between banks, you won't be earning any interest.
I derived this formula that takes into account the lost interest and tells you how many days you'll have to have your money in the new bank before you start making more money versus leaving it in your old bank:
days = (transfer time in days) * (old rate)/(new rate - old rate)
So, for example, if you're currently getting 2.67% and want to know if it makes sense to move your money to a bank offering 3.5%, and the transfer takes 10 days:
10*2.67/(3.5 - 2.67) = 32 days
So that's 32 days the money has to be in your new bank (on top of the 10 days transfer time) before it'll be worth the hassle.