How long does it take a layer of R-30 fiberglass (or cellulose) insulation to become saturated with heat so that it begins transferring the heat?
At the risk of sounding flippant, how high is up? The amount of time it takes heat to penetrate R-30 of mass insulation depends upon several factors, principally the temperatures involved and the amount of convection air that surrounds the mass.
In the test to determine the R-value of the material, the mass is placed between 2 surfaces — one hot, one cold (i.e. 100 degrees, the other 25 degrees) so that there is a 75-degree difference between the 2. After about 1 hour the material reaches a steady state temperature and the calculation is made.
Once the same material is installed in a building everything changes — temperatures, air spaces, convection and radiation, so the R-value and time required to reach steady state is not testable.
Hope this answers your question.