The pic below is of the filtration elements from the two fuel filters just taken off my car. Standard factory type (Ryco Z332) on left, & the CAV type filter element I fitted as a pre filter on the right.
Both had been fitted at the same time about 12000kms ago, including our recent 10,000'ish desert trip.
On this basis I think in future I will change the pre-filter every 10,000kms & the standard filter every 20,000
Neither filter had any water in them.