Battery water timers typically attach directly to a standard garden tap, the ideal place to keep control of your watering system. The batteries for a water timer normally last about 9 months, which is plenty long enough to last an entire growing season. At the beginning of each new season, pop in a fresh set of batteries and you'll be able to forget about it for the rest of the year. Generally you would programme a water timer to turn on twice a day; once in the morning and once in the evening. In our opinion, the Claber water timers are superb and made to a high quality, but the Hozelock water timers are brilliant value for money offering lots of functionality for an excellent price.
If you're setting up a larger system such as a pop up sprinkler system, you may consider using mains powered water timer in conjunction with 24v solenoids instead as these are a more professional choice, and because they're mains powered, do not require any batteries to be changed. A mains powered timer acts as the irrigation controller - the brains of the irrigation system, which then tell the solenoid valves when to activate.