How to find frequency of my sales for each customer?

[Skeptical]

[TABLE="class: outer_border, width: 500"]
<tbody>[TR]
[TD="align: center"]OrderDate
[/TD]
[TD="align: center"]CustomerID
[/TD]
[TD="align: center"]Product
[/TD]
[TD="align: center"]Qty
[/TD]
[TD="align: center"]Frequency
[/TD]
[/TR]
[TR]
[TD]1/1/2012
[/TD]
[TD]EW1000
[/TD]
[TD]Rice
[/TD]
[TD]12
[/TD]
[TD]????
[/TD]
[/TR]
[TR]
[TD]1/2/2012
[/TD]
[TD]EW1208
[/TD]
[TD]Wheat
[/TD]
[TD]22
[/TD]
[TD]????
[/TD]
[/TR]
[TR]
[TD]2/4/2013
[/TD]
[TD]EW1249
[/TD]
[TD]Corn
[/TD]
[TD]8
[/TD]
[TD]????
[/TD]
[/TR]
[TR]
[TD]5/2/2014
[/TD]
[TD]EW1648
[/TD]
[TD]Maize
[/TD]
[TD]34
[/TD]
[TD]????
[/TD]
[/TR]
</tbody>[/TABLE]

I have my data setup like the above. Those are Col-A, Col-B, Col-C, Col-D, Col-E respectively. I have to find out the frequency of purchase for each Customer, in other words, I want to find out the Days between each purchase of a Product from a Customer. I have a drop-down in Col-H tochoose the product.

eg: If choose Rice in the Drop-down, I want to calculate the Days between each Purchase of Rice for each Customer in the Frequency column. I think I am clear explaining this. Here is the sample data link. Please let me know if you need more info on this.

https://drive.google.com/open?id=0B6GD5M1_zN7bNWR3S0NxQ013WjA

Thank you
Skeptical