By default, the TRC program grants both on-demand quota and preemptible quota. If you are able to create a TPU VM with your on-demand quota, it should last quite a bit longer than a few hours. (There are situations in which on-demand TRC TPU VMs can be interrupted, but these ought to be rare.) If your on-demand TPU VMs are being interrupted frequently, please email TRC support and provide the names of the TPU hosts that were interrupted so folks can try to help.
When there is very high demand for Cloud TPUs, it's certainly possible for preemptible TPU VMs to be interrupted frequently. It would be an interesting engineering project to make a very robust training system that could make progress even with low TPU VM uptime, and I hope someone does it! Until then, though, you should have a better experience with on-demand resources when you're able to create them. Reserved capacity is even better since it provides an expectation of both availability and uptime.