When kneading bread dough, how sticky is 'too sticky'?
Having dough stick to your hands is not a sign of failure. 

Stickiness is related to the hydration in your dough, no more and no less. A focaccia dough is going to be very sticky, and it is meant to be. Unless you knead with oil (a valid technique for sticky doughs) you will be cleaning dough off your hands, and a lot of it. At the same time, if you are making a drier dough that you know should not be sticky then it's a sign that your dough either hasn't incorporated the water, the gluten hasn't fully developed, or maybe you need a bit more flour in it. If you've been kneading 10 minutes and it's still sticky then a dusting or two might be required. If you've been very precise with your measurements (you weigh everything in grams, including the water) then this should rarely happen. 

If dough on your hands bothers you then you can reserve some from the recipe and add it in as you knead it, it will keep the dough from sticking and make it easier to handle. 

EDIT: 
From the comments it sounds like you are reserving flour and only adding it after you let it autolyse. This isn't ideal, what will happen is that the yeast will operate on the water and gluten available, and the flour will soak up the rest meaning the available moisture is gone. Adding flour after you autolyse means it won't get incorporated. Add all the flour before you autolyse and don't add any after, except maybe a very small amount for dusting. I'd suggest if its still sticky try giving it a longer resting period or knead with oil instead. 

You are also kneading before you autolyse, which is also not recommended, the right way to do it is to mix your ingredients until incorporated, then rest, then knead. If you knead it before you toughen it up too much.