A DPF is a consumable part. The time between replacement depends on how the car is driven - budget for £500-1000 at some point after 50,000 miles and you will be prepared.
A colleague just had a new one on a Mondeo (£500) after 130,000 mainly motorway miles (car is almost 6 years old), I've known others fail at 50-60,000 - largely 'pootle round town' motors purchased by those who want to save a few quid on fuel, forgetting they are killing the expensive DPF very quickly!