Determining an average time span for a field that changes

[jhurley]

I am quite a beginner and have run into a road block:

I have created a data entry report which shows me what data (by customer) has been entered in by specific data entry employees between certain periods of time. What I'd like to do is now create a formula which shows how much time has elapsed between when the first customer's data was entered and when the last customer's data was entered. The fields that tell me the time and date of the entry are "CreatedOn" and "Created At". What confuses me is that if I use a DateDiff formula for example, the fields that I enter in are the same. The data within them, however, change with each customer.

I'd like to be able to determine:
a) how much time has my employee spend overall entering data (this spans several days so I have to know hours and minutes and seconds)
b) how much time does it take (on average) per customer to enter data (minutes and seconds)
c) how much $ then are we spending per customer (which I would determine from the employee's hourly rate)which is a separate formula that will be easy to create I think.