For how long and how cold should I perform ethanol/isopropanol precipitations of RNA or DNA?
I always did mine at -80 C, but I never compared the results to other protocols (I don't fix what's not broken). But, I was curious about the same thing, so I looked around. I found one paper discussing this: Paithankar and Prasad, Nuc Acids Res 19(6):1346 (1991)

It shows that at low concentrations, EtOH at RT actually outperforms the precipitations at both 4 C and  dry ice/ethanol bath. That difference is quite big at 100ng/ml DNA and lost when there is more than 10 ug/ml DNA. For typical extractions, based on this data, I'd do it at RT.

On the other hand, Hilario and Mackay say in their book that:


  for genomic DNA isolation, different DNA precipitation temperatures
  and incubation times have little effect on recovery rates. One can
  directly centrifuge after adding ethanol without the -20 C incubation,
  and, it -20 C ethanol is not available, room temperature ethanol can
  be used.


They do not, however, provide any citation for that statement.