I want you to think about your day. Your week. Your future. More than likely, you have something planned, especially with spring break coming up. When you made those plans you, human, coordinated with something that cannot be terminated, time. Its almost strange to think about. Almost every action you take is affected by how much time you have and, thus, you must work with, or around, time. For example, you wake up at 7:30 a.m. on a school day and decide to take a shower. You might be accustomed to taking 15 minute showers but, since you are running late, you are only in for 5. Small occurrences like this mark points when humans work with time.
This is why, in my opinion, Hell in No Exit is difficult to accept, even after being there for a while. The characters actually have no idea how long they have been in Hell. They know how much time has passed on earth, but there is no way for them to know about "Hell-time". They don't know if its night time, day time, or even tea time. They're simply stuck in a point that still exists within time but just does not care about it.
Garcin, Inez, and Estelle all had connections with time like other people. For Garcin, there was a time to run. For Inez, there was a time to torture and to hurt. And for Estelle, there was a time to kill. All humans are obsessed with time, most notably THE time. The time to die. So, thinking about this connection, being told to just hang out for no determinable amount of time in a room that has no way of telling the time and is so boring that its nearly impossible to comfortably pass the time would be pretty irritating. And it doesn't help that they're trapped with people they just cannot get along with.