There isn't a standard value but 100uF is what I would use as long as there is enough space, but 10 - 470 is fine.
If you want to be technically correct it is also worth adding a low value disc ceramic in parallel with the electrolytic to attenuate any high frequency rubbish that gets through.
This is because at high frequencies electrolytics are not effective at shunting the interference to ground due to their internal impedance, disc ceramics are good at this and 0.1uF is an accepted value.
Whether you will notice any difference is debatable but it will give you a warm feeling that you have at least tried to do it right.
(If you have anything digital on your board and digital noise is getting into the audio chain then the liberal use of 0.1 disc ceramics across the power supply filter caps can sometime reduce this. There are recognised techniques employed in digital electronic design for reducing this, not all pedal designers are that well up on design techniques.)
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