Shower timers are available in a variety of configurations, including the hourglass sand timer type, and digital models. Relatively inexpensive (in quantity), these egg-timer type gadgets are frequently favored as “giveaways” by water utilities and are entirely passive. That is, they don’t sound an alarm bell and they don’t shut off the shower when they reach their 2 to 5 minute limit. Yet, they can serve as an effective outreach tool and a novel, but helpful, aid to the customer of the water utility. They can be found for sale online by a variety of companies.
Traditional spring activated kitchen timers (with an alarm bell or chime) can be used for the same purpose in the shower, but are a bit more expensive.