DI will run out faster wthout the RO......... I had a friend who used the "Tap Water" DI unit.......... withour water (no greater then 17 ppm TDS) was able to get 300 gallons a cartridge.
There are ways to regenerate the DI resins but they involve some harsh chemicals and if your not used to working with them............. I would not go that route.
http://www.reefs.org/library/article/twp_recharge.html
http://www.reefs.org/library/article/g_deutschmann.html
Cheers,
Vic [veng68]