Here's the thing about open mics, jam sessions, etc.
The hosts control the show. I have been in that seat, it isn't as easy as it looks, and, unfortunately, some people get very invested in a set of rules that seem reasonable, but don't apply. What do I mean?
Players believe in the rule of list order - that is, first on the list, first to play. The thing is, that isn't a rule: The rule is, you play when the host calls you to play. It is legitimate to speak to the host and make your presence known - and your place on the list, if you think that means something - and any other items you hope will influence your performance. But it is at the discretion of the host who plays, and when. The best you can hope for is that list order will influence playing order.
Players believe in 'fairness' but frequently forget how subjective that can be. What seems fair to any one person may not reflect what seems fair to the host, who is responsible for all the performers that appear, as well as being responsible to management to put on a good show (or at least one that inspires everyone present to drink).
Players believe in schedules. They believe the session should start on time, and that there should be constraints on the performances so that everybody gets equal stage time. In 45 years of jam sessions, I think I've been to two that imposed and kept to a schedule, and, amazingly, players still found a reason to bitch and moan.
In the real world, start times are... negotiable; sometimes a member of the host band is late, sometimes there are equipment problems, sometimes it's the sound guy, sometimes its the lack of customers or people signed up to play.
I've grown pretty accustomed to a 4-song standard set. However, different venues have different rules (3 songs, 10, 15, or 20-minutes). Some performers play long songs, and four of them would back things up too far. And some performers alienate the audience, or the hosts, and will get pulled early (aka, getting the 'hook'). And, if a performer is really grabbing the audience, more stage time will be allowed. These last two are the only instances where talent or performance skills affect stage time.
Finally, players are affected by their relationship with other players. If you are new to a particular session, you may be confined to a 'safe' spot, where your unknown skills will not cause a problem for the host or other players. If you are known, there may be players who want you in their set, other players who don't; you may know who the former are, typically, you won't know who the latter are (although it isn't too difficult to figure out). Expect to be 'categorized' by what other performers think is your musical style (hard rock, old rock, blues, jazz, country), and expect this perception to influence when you play and with who.
Last night I went to two open mics. At the first, as one of the earlier people who signed up, I played in the first post-host set, through a few personnel changes, and had a blast. There were a couple of really out-there, unplanned and musically interesting moments (playing a standard R&B tune reggae-style) and everybody was happy - except for one guy, who thought he should have been part of it, and wasn't called. However, the whole thing had started late, because one of the host band members was late coming in from out of town. So it goes.
At the second, a blues jam, the sign-up list was ignored. It also started late, and, although I was signed up for the first set (that's how the sign up works at this one - people sign up in sets, you can see who you will be playing with), I left before I had a chance to play (I would have gotten up five minutes later, but I had stayed as late as I reasonably could). As I was leaving, someone stopped me and said that these people weren't running the session right. I just shrugged - if I can't stay until I'm called, it is on me, not the hosts. Only if I stay the whole time, signed up but not called, do I have a gripe.
Probably the only reason for writing all this (besides my experiences last night, in which I encountered every single one of these factors), is that this holds true for all social interactions. The rules for interpersonal engagement are flexible at best, and there is plenty of room for misunderstanding. A persons expectations have to be adjusted according to the specifics of the situation. General rules don't apply.