Time required for RNA precipitation in ethanol
As far as I know this has never been thoroughly analyzed for RNA, but there is an excellent paper on the precipitation of DNA and the usual conditions in BRL Focus by Zeugin (see below for the paper). Since DNA and RNA are pretty much the same (except for one OH-Group) and the conditions used for precipitation are also similar, I think we can use this for a very close estimate.

Interestingly neither the time nor the temperature of a precipitation have a great influence on the outcome - they change the yield only by a few percent. If you look on figure 2 from the paper mentioned, you see the following:



This subfigure shows that the recovery is almost not influenced by the temperature, but only by the conconcentration of the DNA (or RNA). Depending on how much RNA you expect (how much cells did you use for the isolation) I would add a co-precipitation agent when you have only very small amounts on RNA. Glycogen works very well here. Otherwise I wouldn't worry.



This figure shows that the incubation time has only an influence on the percentage of the recovery when then time is too short. If you incubate for an hour, you are on the safe side.

Reference:


Ethanol Precipitation of DNA.