It depends on how fast of a swimmer you are.
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Well considering that there are (counting on fingers...) 18 events (13 individual events and 5 relays) and a male and female record for each, and then a long course meters set of records and a short course meters set of records, you see that this is a large number of times to update. (72 records) I suggest you take a look at FINA's official record page
http://www.fina.org/swimming/records/index.php
or browse Wikipedia. I will not post WRs in an uneditable post, because 2008 is an Olympic year and every WR is breakable this year.
a swimmer should be a able to go 25 meters/yards (1 lap) in less than 30 seconds. although most competitive swimmers do it in 15-20 seconds.
The fastest recorded swimming is 2.35 m/s (roughly 4.5 mp/h), this was recorded in 2008 by 2 people.