BB Software :: How Often Does BIS Check-in With Exchange

I'm sure this topic has been covered, but I cannot find the whitepaper on it. I have a client that has numerous employees with blackberries, but they refuse to shell out the money for a BES. So unfortunately I get to support them with BIS. I thought I read somewhere about the check-in process to Exchange and having to re-validate the settings either through the device or the BIS website. So to put it simply, is the BIS attempts to checkin with Exchange and finds nothing, how long before you have to go through the process of re-validating?