Substrate: It depends on what you use. I use newspaper and change it every time they poop/pee. This is more work than spot cleaning with aspen. Aspen should be swapped out completely every couple months (someone with experience, please chime in on an exact figure). Two weeks seems too frequent.
Water: No need to change it everyday unless they soil it. I do a complete swap out with clean inserts once every 7 days (or immediately if they soil it).
Hope that helps!
JonV