Are we talking deicing or anti icing-
2 very different subjects with there own requirements, though both are normally carried out at the same time that is certainly not always the case in all parts of the world or airports.
Deicing is at it says-removing the contamination from the aircraft-thats it.
Anti icing however is a different ball game with many variables, ambient temperature, weather conditions-ie rain, sleet, snow etc, temperature and mix of the fluid being applied and of course the type of fluid-there not all the same-this will directly effect the "hold over time"...can be minutes to hours...once the hold over time has expired you have to "reapply anti/de icing fluid again if the weather conditions/aircraft require it.
And afterwards you need to wash the aircraft as its leaves a sticky mess...
A couple clarifying remarks:
Type 1 de-icing fluid also has different hold-over times dependent upon temperature, precipitation, mix, and airframe composition, to name a few; but Type 1 fluids are not anti-icers, they're de-icers.
You might also have to re-spray an aircraft if the conditions worsen, and in some cases much, much sooner than the hold-over time for the original conditions. The hold-over time, while based on science, is not a guarantee and the fluid can fail well before the calculated hold-over time if conditions worsen. For example, if you spray the aircraft in very light snow conditions, but on the taxi to the runway the rate picks up to moderate or changes to light freezing rain, you might have to go back to the pad well before the original hold-over time. It also depends on if the fluid was mixed and applied properly. I've seen it where the fluid failed well before the hold-over time because it was not applied thick enough in the first place, all because the contractor was trying to stretch their supply to save a buck or two.
Finally, there is no specific need to wash the residual fluid from the aircraft after flight. It will naturally come off in its own time. Perhaps a person will do so on their own private aircraft for aesthetics, but that's it.