Do you mean a Foley catheter in the bladder?  Usually every 2 to 4
weeks when you're not in the hospital, or more often if there are
problems, like leaking, or if they get crusty or clogged.  Who says 6
to 8 weeks?  

--Patti

It partly depends on the type of catheter (material used in manufacture).
There are short and long-term catheters. A long-term in-dwelling Foley-type
balloon catheter could last several months. In the UK we leave well alone
until there is a problem, such as blockage or leakage. So catheter failure
determines longevity. A patient who drinks plenty will tend to have less
encrustation which causes catheters to block eventually.