Where I work they aim to give it by 4nm, but will tell us late landing clearance if under 2nm. As far as rules for how far to go without it, I am not aware of any unless operator imposed. In the light twin I used to fly we would fly level along the runway until we got it if it was suitable to do so (ie. vehicle vacating at the far end), flying something heavier now there is no rule but I wouldn't really want to continue past 50' at the latest unless I was 100% sure of getting it, and in reality depending on the circumstance would call a go around earlier than that anyway.