counting days between dates

I got another request from marketing....

I need to count the days between the FirstOrderDate (column 1, datetime datatype) and the LastOrderDate (column2, datetime datatype), which will give me the LifeTime (column3), which is the total number of days the customer has been ordering from us.

What's the quickest way to do this in SQL?

thanks in advance for any help.

SELECT DATEDIFF(day, firstorderdate, lastorderdate) AS no_of_days
FROM [table]
GO