I need to repair the Nikon's SB-600 speedlight. As I need to access to only the bottom part of the speedlight I skipped the danger of electric shock. However when I tried to disconnect one of the socket I get the shock to my finger. As I have no resistor I put the speedlight in the closet for about 24 hours and now I'm wondering: how likely that the capacitor is discharged to safe level?
A properly formated electrolytic capacitor almost has sort of an imprinted charge (like electret condenser microphones) and will lose very little charge by itself since the voltage is what actually causes it to maintain its isolation (which is why you should power up flashes for at least 15 minutes every 3 months or so if you don't use them or the isolation will deteriorate, greatly increasing the danger of breakthrough when finally powering up the flash, and breakthroughs are often terminal). It very much depends on the rest of the circuitry how much charge will stick around if this is a flash in active use (some flashes will discharge controlledly in few minutes, others won't).
Note that using an 1/1 manual flash is not guaranteed to get rid of all charge: flashes cannot utilize every bit of charge and may maintain reserves for preflashes, red-eye flashes, prefocus flashes and what other uses are there.
So in general be very very careful until you actively discharged the capacitor, and don't underestimate the stored energy when doing so as it can act rather destructively. If you have the schematics, there often are terminals reasonably safe for discharging since they have circuitry limiting the surge and dissipating the energy. If you cannot read schematics, are you sure you are the right person opening a flash? Some wand flashes store 90J or similar. A 100th of that energy will give quite a jolt to horses touching a medium-size electric fence.
Surprisingly, but after three days the voltage on the board connector was still more than 200 Volts! So I have to state that the capacitor have very low self discharge current.
I connected the resistor directly to the board connector as I could not reach the dedicated contacts on the flash head.
I have personally pitted screwdrivers on (non-photoflash) electrolytic capacitors that had sat charged but idle for WEEKS.
Self discharge specifications of such capacitors will always be worst-acceptable numbers, since self discharge and leakage currents are usually considered undesirable to an engineer.
Never rely on a big capacitor being safe unless you either a) received it obviously shorted, b) discharged it thoroughly, c) measured the voltage across its terminals.