With Tobi, we're usually able to adminster 12 hours apart -- 6 a.m. and 5:45-6 p.m. Though on weekends it usually closer to 10-11 hours apart. 7:30 or 8 a.m. and 6 in the evening.
With albuterol and atrovent (duoneb) we wait at least 4 hours between treatments and DS does 3 per day -- 5:45 a.m., 5:30 p.m. and 10 p.m.