How long should bread dough warm up after cold fermentation?
...as long as it needs to, depending on a wide variety of factors.

You can shape it for baking, put it in the fridge, let it rise, pull it out of the fridge and put it in the oven - no warming up time at all. 

If you are going to punch down/knead/form after it comes out of the fridge, you can do all that cold, and let it rise as long as it needs to before baking - which will depend on the dough and the room temperature.

Or you can let the bulk dough warm a while.