SQL & PL/SQL

I am facing the following problem:

I have a TABLE like this:

CUSTOMER_ID (number),
EVENT_TIME (timestamp),
IND_PURCHASE (number) # indicator 0|1 if buy

I want to calculate the time(in minutes) since the last purchase of each customer. How can I do that in PL/SQL?

Basically for each CUSTOMER_ID at each EVENT_TIME calculate CURRENT_EVENT_TIME - (LAST_EVENT_TIME where IND_PURCHASE = 1)

Your help is greatly appreciated.