How long should a dementia patient stay in one place? How long in a wheel chair, bed, a lounger?

If she can't walk are you talking about transferring her from one spot to another?

People who are immobile should be repositioned every 2 hours and their weight redistributed at the same time. For example, if someone is in a wheelchair with the weight off of their right hip and leg, in 2 hours they should be repositioned but with the weight off their left hip and leg this time. People use foam wedges to keep the weight off of parts of the body, some use pillows although pillows tend to gradually return to their shape over time and aren't as beneficial as wedges but a pillow is better than nothing when keeping weight off a part of the body. And the pillow or wedge should be placed so that the space is tight, that is so her weight isn't bearing down on the pillow or wedge either. Place it so the hip/leg/bottom are off the seat of whatever she's sitting on. It should feel like a tight fit if you've got the pillow or wedge in there correctly. 

So reposition her every 2 hours and get the weight off of her bottom, hips, and thighs. For someone who is immobile pressure ulcers are your worst enemy.

Varies but it is highly suggested that they are turned every 2 hours maximum to prevent bedsores.