How long can an aquatic turtle live on land?

I have a 9 month Red-eared Slider. She was in her tank. I realized that she was not in there Sunday. What I thought was her over the days was actually her land floater. I've looked all around my house. I have no idea where she could be. I just wanted to know if I have a chance that she is still alive. If so, How can I catch her?