how to I set up the following duration formula?

Can you give some example situations and calculations?

If event A starts at 8am and ends at 1pm the volunteer hours for that event will be 5 hours.  How do I get it to do the calculation to get the 5 hrs?

I suggest that you have two time of day fields types for input.

Then ToHours([end time]-[start time])

You may also want to do
if(not IsNull([end time]) and not IsNull([Start Time]),ToHours([end time]-[start time]),0)

How do you handle this situation if the [date created] is Date/Time and the [Date Finished] is just a date? I don't think I can change the Date Finished type at this point without messing up a lot of data. I just need the amount of days from start to finish.

np, convert the date/time to be a just date for the purposes of the formula

[Date Finished] - ToDate([Date Created])

The above would be a duration or

ToDays([Date Finished] - ToDate([Date Created])) would be the number of Days ie a numeric result.