I face a great challenge in knowing what exactly bleed should I use in my document with A4 and another in A5. Actually, this will help me to guide printers on what should they suppose to crop.
There's no "exact" amount of bleed.
There's often a minimum which can vary, but generally if you use .25", 1p6, or 3mm (as @SaturnsEye suggests) things are fine.
Print providers use crop marks to know where crop. They do not measure in from the outer bleed edge. The size of your bleed is largely irrelevant as long as some bleed exists.