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How often do turtles lay eggs? and if she is in the water all the time how do I get her to be able to dig a nest? How long can she be out of water?

That, in some part, depends on the turtle. Assuming she's old/big enough, if you see your turtle becoming restless, trying to dig with her hind legs, and showing signs of wanting to get out of the tank, she's likely to be gravid (has eggs to lay). She may also lose her appetite.

If you have a backyard, you could put her out and see if she picks a place to dig a nest (if the yard's not securely enclosed, watch her so she doesn't wander off). Or, you could build a nesting site for her. There's info elsewhere on the site on how to do it. They can be out of the water for a while. Painteds come up from the pond near the house and dig nests every late spring/early summer. It generally takes them a few hours from start to finish to do it.