Purchased African Side Neck turtle (age 1 year). Question-how long can they hold their breath underwater? Are these kind of turtles more underwater kind?

You really should have done your homework before buying a pet that needs so much care.

Use this google search phrase to see a lot more info.
All to know about african side neck turtle

http://petcaretips.net/african-sideneck-turtles.html
http://animal-world.com/encyclo/reptiles/turtles/AfricanSideneckedTurtle.php