Water will last ages! It won't evaporate as it can't escape anywhere as it's a sealed system.
If it does start to leak, the DOT fluid you used before might've damaged the seals, but it should be okay.
The only problem you will have with water is in winter, when it freezes and jams your brake on! All you have to do before then is add some anti-freeze or vodka in with the water, then re-bleed.