Life of the used tubes

Can you use the tester to test the used tubes and determine the life left in them?
tube life depends on the circuit design that uses them, among other things.
Depending on the tester you can test for shorts, and compare transconductance to new and minimum good, but you cannot tell how long they will last in hours. If you have lots of experience w that unit and that tube, you could guestimate.
Swampwalker is absolutely right. It depends on the circuit the tubes are used in. I recall old Jadis gear in the 70ies, which were absolute tube eaters.....
Yes you can, sort of. I have an overhauled TV12 tester and have been experimenting with how tubes behave as they wear. I measure new ones and then periodically check them as they are being used in various gear and data log everything for each individual tube.

In my experience, little 9 pin tubes like 12AX7, 12AU7, etc., will lose transconductance (gain) as they age. However, the big power tubes like 6550 or KT66 or even EL34 will tend to high leakage as they wear - in other words, they actually fail from high leakage rather than loss of gain. So which mechanism dominates depends on the type of tube, but the rate of wear will depend on the circuit in which they are used.

Between my system which uses 16 tubes, and my local buddies which have tube-based systems, it seems that low-gain tubes don't sound bad at all so they effectively last a long time. But this doesn't apply to the power tubes which noticeably degrade after a year or two of use but still test fine gain-wise.