Also, make sure you timestamp is long enough so that you're getting as specific as your needs dictate. Here's some info on timestamp definitions if you're not familiar. As dragonchild suggested, the manual is always a good place to start.

Updated in response:
Something along these lines might help... 
SELECT (SUM(exittime) - SUM(starttime)) / COUNT(*) 
AS average FROM cust_records;

[download]

Thnx to dragonchild for pointing out my incorrect logic :)
~hb

In reply to Re: MySql Timestamps by heroin_bob
in thread MySql Timestamps by Agyeya

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