Abandonment

[LDUB]

I know we have went over things like this before, but I'll ask anyways. I had this come up a few days ago. So, R1, 2 out, 2 strikes. Batter swings at low pitch that is not caught by catcher. F2 picks up ball and is looking to tag B1, but B1 is already about 15 feet away walking towards the 3rd base dugout. So I look at R1 and he is crossing the mound on his way to the dugout. I know when to call the BR out, but how far do you let R1 go before you call him out for abandonment?