One airline had a 1hr 30 minute time frame. If slot was outside this time, crew were able to refuse boarding until a slot improvement. However sometimes the slot was 1hr 35min from STD and moving around. As mentioned its a catch 22 situation. Ideally gate staff want pax to stay nearby so as to provide info or for quick boarding. However many pax would just rather go straight to the bar for another pint!
Might we ask what other options the original poster would suggest in this situation? We all know of the various scenarios which would develop. Things aren't always so clear cut when you are pax!