- it is really not possible to guarantee the characteristics of the capacitor merely by testing only a few random samples from the unknown batch.
- If these parts are going into a design in to the field, I would never advise to do that.
Capacitor voltage ratings are not highly controlled parameters. So it is difficult to gauge the voltage rating of all the capacitors just by testing.
For example: this was quoted by one of the reputed capacitor supplier. When we order a capacitor with 5 V rating, the capacitor supplied to us can be 5V rated one, or 10 V, or 16V or even 25 V. It depends on their production line and distribution line. Sometimes 5V rating capacitor doesn't even be produced separately but will be listed as 5V(and delivered at slightly lower price) just because there is a competitive part outside with same rating.
How much time is enough?
Really depends on your application life time. Normally, endurance testing (HALT - Highly Accelerated Life Rest) can be done to test the life time of the capacitors. Ceramic capacitor keep their charge well up to high temperature (100 degree C or more) and hence I think the test would be difficult!!)
I believe you are not planning to test all of them. Best bet is to buy new ones with part number, unless this is not going to a product to the field.
From Murata, there is one simple description for the measurement of life time for the Ceramic capacitors in the below link
Below is a quick comparison of DC bias characteristics of four capacitors of different voltage rating but of same value, same package and same temperature tolerance and same accuracy.
GCM188L81H104KA57,DC0V,25degC ---> 50 V DC
GCH188R71E104KE01,DC0V,25degC ---> 25 V DC
GCJ188R72A104KA01,DC0V,25degC ---> 100 V DC
GCJ188R71E104KA12,DC0V,25degC ---> 25 V DC
I would never try to attempt to gauge the voltage rating only from the DC bias characteristics. it is simply not defined but I believe it will follow some what the higher voltage rating curves only based on my past experience.