As a pilot and mechanic who's flown or done ground runs on radials (from R670s on through R2800s) on not sure I've ever waited more than about 5 mins for warm up. And warm up is usually not done at idle. About all you're looking for is oil temp because the oil is usually a heavy, single viscosity, like 50 weight, and if you throttle up too much too soon pressure will get too high popping gaskets in some areas while other areas will have low oil volume. Roller motors like the R2800 can actually handle low oil flow surprisingly well, though, because they don't rely primarily on plain bearings.