My current one was meant to last 5 years - the term of the loan. However, it looks like I will need to keep it rather longer and take a chance that repair bills aren't too bad.
If I had more choice (financially), I would buy brand new and change it at either three or four years. Three years due to MOT or four years as it's after that when capital allowances start diminishing more. If mileage isn't excessive, I reckon it's reasonable to get it through the first MOT without much (if any) cost.
As things stand, I bought my current ont at a bit under three years old and will probably sell it on around ten years old (if it lasts that long).