Capacitor Discharge
There is no general answer to the question about how long your capacitors will last under shorted pulse discharge conditions because capacitor life depends strongly upon the type of capacitor you are using, the voltage stress, the discharge current and energy, and the nature of the external curcuit. Certainly, the first thing to do is select capacitors rated for pulse discharge conditions (like photoflash and laser energy discharge capacitors).
In general, capacitors designed for power supply filtering applications are not well suited to pulse discharge conditions because, among many design factors that are beyond me, the internal connections between the capacitor plates and the terminals are not designed for the high currents that occur during quasi-short-circuit pulse discharges. If they don't fuse instantaneously (which, obviously, yours have not), they will go through damaging thermal cycling leading to premature failure. They may also have (relatively) high internal resistance or inductance leading to peak current limiting or pulse lengthening.
Capacitors rated for higher ripple are not necessarily suited to pulse discharge applications (although they may be better than those not rated for ripple current) because the ripple rating has more to do with the losses within the bulk of the capacitor and thermal transfer out of the bulk of the capacitor, not with the instantaneous peak current capability of the internal connections and punch-through resistance of the dielectric
If you can find it, a fascinating series of books is the 4 volume set, "High Speed Pulse Technology," by Frank Frungel (Academic Press). All 4 volumes would be very interesting to this crew, but Volume III, "Capacitor Discharge Engineering," is directly applicable to this question.
One of the critical factors affecting capacitor life is the presence or absence of voltage reversal (even of extremely short duration) across the capacitor dielectric resulting from oscillatory discharge (which is not trivial to detect or avoid). Exact external circuit characteristics, controlled by stray inductance, wire length, etc., strongly influences the oscillatory nature of the discharge circuit. The existance of oscillatory conditions may not be obvious in the absence of careful evaluation in the lab with a sampling scope. Discharging with a hard short (like a screwdriver) is highly likely to result in oscillatory discharge due to the relative absence of an energy dissipating element in the discharge loop, along with the presence of stray inductance.
Have fun, and wear safety glasses when playing with capacitor discharges.
awright