This shower beeper has really helped shorten showers in our house. The timer cost under a tenner from the online retailer Smart Green Shop.
I was after a very specific design of shower timer - it had to work with the press of a single clearly labelled button, it had to beep, it had to attach securely. I knew if I got a non-beeping version, or an overly-complicated one, it would end up as a well-intentioned shower ornament as it wouldn't get routinely / effectively used. Hence I ended up with the above design. If you are less picky than our family then it would be worth checking with your LOCAL WATER COMPANY as they often have shower timers to give away for free (ours were offering the sand-timer i.e. silent version, so we didn't choose that option).
Those with eagle eyes will notice our timer is dangling from a ribbon - the suction cup doesn't like our (slightly textured) tiles and reviews Amazon warn that the timer is prone to falling off the wall. My solution was to loop a ribbon around the suction cup. I have a stash of 'free' ribbons, as I always cut the ribbon tags from clothes - those ribbons (which are supposed to keep clothes on the hanger) always end up looking messy poking out from your neckline, if you leave them in situ. These 'free' ribbons are ideal for crafting and for doing jobs like this.
If you have read my 25Home blog posts then you'll know that I like a clutter-free shower area, as anything that gets repeatedly wet, inevitably ends up slimy and unpleasant. Our shower timer is hooked high up, well away from any water splashes, but in a position making it impossible to miss when you get in the shower.
So, start timing your showers and immediately start saving money and the planet.