If the fall is towards the down pipe than the gutter shouldn't hold any water at all.
That depends on a number of factors.
1. The roof area that is draining into each gutter,
2. The number and diameter of each downpipe,
3. The volume of rainfall at any particular time.
Modern gutters are slotted along their length to permit overflow when the drains are unable to cope with the volume of rain falling. Our house was built around 1920 for a guess, but sometime years ago, the gutters were replaced with box gutters, around 200 mm deep. The problem is, they didn't increase the number of downpipes (which would have been easy with old homes because they usually drain straight into the unkerbed street gutters), so in very heavy rain, those overflow slots are working overtime.