Assuming you mean the plastic vent lids, the polycarbonate covers should last much longer than the original, regular plastic lids, which typically only last a couple of seasons, if you're lucky. The lids on the Fantastic Fans are polycarbonate, and ours has lasted since we purchased our Airstream new in 2005, while I just installed the third replacement, regular plastic lid on our other vent.
I intend to install a polycarbonate replacement lid when this latest one goes bad in a year or so, but I already had a replacement in the closet. They fail so frequently, that I always carry a spare. (Polycarbonate replacement lids only became widely available a year or two ago.)
In any case, we are considering adding a second Fantastic Fan, so this may not be a continuing routine repair. However, as our original Fantastic Fan ages, we may need a replacement polycarbonate lid for it; although, it still looks structurally sound after 4.5 years.