Depends,easy to see? Easy to change. As already pointed out.
On my aclass w 168. Its worked out at 6 yrs.the pulleys are a real pig the belt if coming folded is little better!!! So for me its a warm ,dry problem or a cold,wet,dark.problem ,with a likely added recovery cost.therefore a no.brainer.
I would say ,for cost ,if doing yrself just do it .then u can forget it for at least,in my view 6yr..
My old pulley bearing were dry noisy and the belt had a number of cracks..