OK, I’m exaggerating a little. Let’s make that 5 years.
Whatever the exact timeframe, there is no doubt that any given look and feel for a website eventually grows old.
As a freelancer, or for any business, you can’t allow that to happen. You can’t have a website that looks like it was last worked on back in 2005 or, even worse, 1998.
In some ways it’s odd that website design should be so susceptible to changes in fashion. If a particular look and feel works, why change it?