ScrubAlert

ScrubAlert is a free, non-profit utility that reminds you to wash your hands after you've taken a specified number of steps. Translated into 30+ languages and fully open source, ScrubAlert can have other utilities, like reminding yourself to hydrate after step intervals for example. 
Step count is a far more effective estimator of bacterial exposure than a simple timer and as an unobtrusive solution for helping kids and adults with hand sanitization practices, Scrub Alert can assist you in staving off unwelcome germs.
Using the health app's native pedometer, battery impact is extremely minimal, only querying steps every 15 minutes as a background fetch process and issuing a notification when necessary. The app has no connection to the internet and saves/tracks none of this data. When it's time to wash your hands, a built-in 20 second timer helps encourage lasting hand washing habits.