The only way to really predict whether your tires can make a long trip is to keep a tire log from the time the tires are new.
Before mounting a new skin, I measure the amount of rubber I've got above the wear bars and record it.
Periodically throughout the tire's life, I measure the amount of rubber I've got above the wear bars and record it, along with the mileage on the tire at the time of the measurement.
Unlike many here, I've found that the tires wear in a roughly linear fashion with mileage, with tread wear slowing down a bit as the tires approach the end of their usable lives. Since I hardly ever ride into the wear bars, I can't vouch for the fact that the wear becomes more rapid here, as many have observed. Anecdotally, the very few times I've ever done this, it appeared to be true.
With my tire log, I'm now armed with a rough gage of tire wear per mile. Now, I can make valid predictions on whether a tire will make the trip. The system works well.
I need this system because very little of the miles on my big street-burners are of the "around the flagpole" variety.
I can't afford the time it takes to get a tire changed while I'm on a business trip. So, the tire log system works well for me.