We get asked this a lot, understandably, and what we advise is that your car should be returned in the condition that you would expect a car of (x) many years old, that has done (x) miles, at that stage.
Many finance companies have their own slight variations of what fair wear and tear is and you can ask for clarification on this from the relevant funder before you agree your deal however as a rule of thumb all vehicle leasing companies do follow the 'BVRLA' (British Vehicle Rental Leasing Association's) fair wear & tear guidelines which you can view by clicking on the below link.
Click here to view the BVRLA guide