What is the shelf life of Ebay replacement Prius battery modules?

Measure them and or balance them as soon as you get them. Wait a week and measure them again.

No one can tell you how long any one module or set of modules will last, unless they are new. And even those could have slight variations within the set.

Many threads here are about after market used modules, like this recent one
Dr. Prius Red Bar | PriusChat

 

Prius NiMH modules self discharge over time... They can go down to near 0.0volts after many years and still charge up and be fine, but once they hit negative voltage / polarity reversal, they'll charge up and recover, but won't ever perform very well again.

In general, there's a huge amount of crappy modules on Ebay that will fail right away... But this dude's modules are super high quality because of all his expensive equipment and decades or expertise: Hybrid Car Battery Distributor | Contact Us | 2nd Life Battery

 

There are a couple other things besides reversed polarity. Uncompressed bulged while charging modules that have returned to what looks normal in size and electrolyte leakage. Sometime those things are impossible to detect from other healthier modules. Have you noticed any other module killers?