I just tested a Samsung LED, and I got very different results.
I would agree that CCFL LCDs take forever to warm up. You may still get meaningful increases in luminance 30 minutes after turning it on. However, LED LCDs--at the least this Samsung--are much more stable than the SpectraCal test suggests. An hour after turning it on, its luminance was down only 2.8%, which is a negligible change.
More interesting to me is the change in chromaticity, which, although small, is probably worth considering.
In my opinion, 30 minutes is adequate warm-up time, but this is obviously a matter of judgment and practical time constraints.