How to limit bathroom shower water usage by time?
They make a pretty cool device that installs on the shower arm upstream of the shower head. It's a battery operated valve that cuts the water flow back (to like 1/3 flow) after a pre-programmed time. It doesn't shut off, it cuts the flow back enough to make for a very unsatisfactory shower experience but provides enough flow for her to finish rinsing off. It will basically force her to leave the shower.

The time duration can only be adjusted by applying a special magnet from the outside, so no one but you can adjust the activation time. it is not cheap (around $90 US) but the reviews are positive. I don't normally endorse, and I do not own one, but I have never seen another product like this: showermanager