working out customer purchase latency

[KNKN9]

Hi,

I am trying to find a formula that is able to help me solve the average number of days between repeat purchases at customer level.

Data

A B
Customer ID 1 21-11-12
Customer ID 1 09-12-12
Customer ID 2 21-11-12
Customer ID 2 24-12-12
Customer Id 2 31-12-12

If someone could help me come up with a formula that would be much appreciated.

Thanks
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[Gerald Higgins]

With the data posted, what should the results be ?