that's what I usually do, set duration and compensation expected. If boy cuts me on time I usually pay what we agreed for but tell him firmly I'm not happy and if on next visit to his bar he is trying to reacquaint, i tell him discretely but firmly that I'm not interested in anymore and explain why.
Long time for me is till breakfast, whenever I decide I wont to eat it and if i engage for long timer it's how I ask " do you stay with me till breakfast ? ", works better than simply asking for long time