How many years do you assign a new desktop \ laptop machine with regards to life expectancy?

With regards to future budgeting I would have thought that for a business class machine for the standard office environment it would be roughly:

Desktop Life Expectancy - 4 years
Laptop Life Expectancy - 3 years

Would be really useful to know what others think about this. Of course its estimated but still provides a way of forecasting future expenditure.

Thanks for all feedback!