I just installed a solid wood door (first time doing it) and I got a gap of 3-4 mm on the hinge side and 6 mm on the striker plate side.
Just looking at it I think the gap is a bit too much, but I might be overthinking it. I even surface mounted the striker plate (instead of inset in a mortise) just to reduce the gap a bit.
The hinges are properly inset but I think that I can shim between the hinges and the door jamb with some cardboard shims.
Best Answer
The two primary factors are latch operation and weather sealing (where applicable). 1/4" (6mm) is getting fairly large, but still within what I'd consider acceptable. It's hard to say without seeing it all in action.
Much more than that and I'd take your approach of shimming the hinges, but use some nylon or other non-biodegradable material in the case of an exterior door.
You could also surface-mount the strike plate, but this does look a bit less professional.
A less significant factor is the appearance of a large gap. Generally, as long as the gaps are individually uniform (parallel), they can be what they are. Varying gaps are what look sloppy.