Is there a way to keep apples from turning brown once sliced?

A bit of lemon juice in water. If you do not like the slight taste, rinse and pat dry just before cooking.

Agree that it matters what you are using them for. Lemon juice will work well if you are making stewed apples, butter or apple sauce but not so good for baking.

For what application and how long? Lemon juice works but is really strong. You can soak in strongly acidulated water, then pat dry to use. I supposed sprinkling and tossing with acorbic acid will do (or crush up a tab of Vitamin C.) Use Granny Smiths, they don't brown as much.