I can personally attest to nearly an hour's worth of travel before the system alerted me to a lost sensor.
About 45 minutes after leaving the campground one morning I noticed I had one tire that was not giving the same readings as the others. It was showing the same readings as it had when first leaving the campground nearly an hour before. I was looking for a place to stop and check things out when it beeped and showed no reading for the errant tire. Before I could stop and look at it, the next check showed the tire sensor had regained contact and now showed the same temp and pressure as the other tires. It had been out of contact for way over 30 minutes with no notification of that fact.
An easy way to check the length of time is to simply leave the monitor on and drive away from the trailer. It will be nearly an hour before the monitor alerts you that it has lost contact with one or more of the sensors. That's a long time.