So I got caught stealing from Hancock and now he wants to kill me. How londg should I stay out of that city for him to hopefully forgive me so I can interact with him?

I don't know. You want a guess?

Have you tried sheathing your weapon around him? He may never return to normal if your Goodneighbor crime level has reached attack on sight. In Skyrim your could go to jail or pay a fine to lower your crime level. I haven't seen that option in Fallout 4, though my player doesn't steal anything. You also might benefit from a very long vacation from Hancock, but I wouldn't know how long. You could test by finding some chair or bed outside of GN and waiting/sleeping for a week or more and returning and seeing if his disposition has changed. You could *fix* this from the console.