Anything that can be designed can be designed forever. Even simple items such as a spoon or fork can still be designed further, even though they've existed for hundreds of years and have gone through so many iterations. So when you need to finish a design for presentation, how do you know when to stop?

Time might be the #1 reason, but what if you were designing something on your own time as a hobby?

I think for me it's when I cannot immediately think of any more major issues that are of immediate concern.