You need something that will make the shower icy cold after 5 mins running - that'll get her out! 
I used to stay in Abu Dhabi hotel for business and they had a notice on the shower about water use and it's something I still do today.....
Enter shower and turn on - shower with plain water for max 30seconds to rinse off body.
Turn shower off.
Apply shower gel/soap. Lather well.
Turn shower on - Rinse off and wet hair, Max 30 seconds.
Turn shower off.
Apply shampoo. Lather well.
Turn shower on and rinse for 30 seconds max.
There you have it - the 90 second shower that doesn't waste water and doesn't waste half the shampoo/shower gel down the drain either!
Maybe you could make a timer with led lights (like traffic lights) to do the visual warning and also have a motorised valve to stop/start the shower water???
I still prefer the icy cold option though - that would be you tube stuff! 