1/ FARs prohibit takeoffs with snow ice adhering to the aircraft.
2/ Once an aircraft has been de-iced, the time interval before it must again be de-iced is based on many variables, such as:
* Type of precip falling (snow, ice pellets, freezing rain).
* Intensity of precip (light, moderate, heavy),
* Type of de-ice fluid (Type-1, -2, -3, -4),
All of these things (and others) affect the effectiveness of the de-icing fluid, since the precip (water) is diluting the fluid. This effective time of the fluid is called "holdover time" and it's really brief (2-3 minutes) in some freezing rain conditions, and much longer for dry snow conditions.
After USAir lost 405 at LGA back in 1992 (F-28 on takeoff, iced up) Faa held a major industry conference on de-icing, and lots of loopholes and gray areas were tightened up, and folks have a better understanding about the threat(s) if procedures are properly followed.
ALL views, opinions expressed are mine ONLY and are NOT representative of those shared by Southwest Airlines Co.