Probably a usable amount of time I guess - I expect it probably takes 2 or 3 seconds to fully comprehend, then that leaves 5 seconds or so to take cover.
From someone living in UK where earthquakes are rare and tiny, what sort of cover can you take in less than 10 seconds? Go jump in a steel bath tub or something? Stand in a doorway? (Is that a thing? Does it depend on wood Vs brick Vs concrete?)
You grab your child practicing their walking skills on a dangerous edge.
I just get up and stand in a doorway away from the unsecured bookshelves.
The bigger effect maybe is that prepares you mentally for the impact, so you're already planning your next steps and aren't in quite as much surpise/panic as if you had no warning.
When things are collapsing, seconds can be the difference between life and death. Even if a system like this only saves a few extra percent of lives lost in a major earthquake, it sounds worth it to me.
Depending on how earthquake-proof a building is, this could be bad advice. If you're in an earthquake-proof building, a table will protect you from falling objects. If the building you're in collapses, you'll want to be next to something that's not very compressible, like a bed or a couch. The collapse will leave open triangles next to objects like this, which are the best place to survive a collapse.
In earthquake country we don't have brick or unreinforced concrete doorways, for this exact reason. Visiting brits tend to make fun of our flimsy wood-frame buildings. Having actually been under a doorframe in a magnitude 6.9 earthquake, I'll take the wood frame any time ;)
I'm not going to interrupt may day for a 4.0. For a 6.0+, I will probably turn off anything on the gas stove and walk outside if I can.