There are no rules for when you drop a coalition. You can do it anytime you want. It's up to you.
Even if their base is locked out, anyone in your coalition can enter your base as long as you have it set so they can. It's possible that there are a few active players still, but their leaders are inactive or don't want to pay the rent for some reason. The longest a leader can go without being auto-demoted is 45 days.
�Let there be truth, happiness, and waffles�
-Vagabond, Dark Lord & Avatar of Gnarr
The Justiciars
Make a judgment call. I wouldn't drop them immediately, but if our coalition list was full and I wanted to add someone AND we had some defunct bases on the list - that's when I'd be breaking coalitions.
Paragon City Search And Rescue
The Mentor Project
I try to keep track of Coalition leaders through globalfriends so I can keep track of how they are doing, or for one or two of them throw a toon in to monitor from the inside >.> . Unfortunately I seem to forget who runs what and have to blindly cleanout the list once in awhile for new ones anyway 
, unpaid rent is a great 'tell' though for how the SG/VG is holding up, if it's seemingly red every time you go to enter your base for a little while I would consider breaking it if I need the space.
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Just a note not sure if it was fixed yet but you had to break coalition manually with the / command the option in the coalition menu wasn't working for awhile.
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Are there any guidelines for maintaining coalition links with seemingly dead SGs? I mean, my SG has coalitions with with other SGs that have the 'can't enter, upkeep due' sign on the base entry window.
Can members of the closed SG still use our teleporters and stuff? Or should I assume the other SG no longer has any active players or they would pay the rent if they were playing?
How long should I wait before breaking the coalition to allow room for other coalitions?
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Can members of the closed SG still use teleporters: Yes, if your permissions allow it. The status of their base has no impact on if they can use yours or not.
How long to wait: Purely a judgement call on your part. I worked for a while to try to find out who was running two SGs coalitioned with an SG whose leadership I inherited years ago - since, for all I knew, it was a small family-type SG that didn't even have a base or some such (which will also show "cannot enter," IIRC.) If I can't find/can't get ahold of them - the coalition gets broken.
I do wish, especially with the "superleader" now, that the SG registrar would show contact information for that person - or even just give a generic "SG Global" to go to whoever has the lead, so if DeadCoalition is showing as red/unpaid, you could just email, say, SG@DeadCoalition and see if you get a reply from their leader at all.
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I try to keep track of Coalition leaders through globalfriends so I can keep track of how they are doing, or for one or two of them throw a toon in to monitor from the inside >.> . Unfortunately I seem to forget who runs what
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ouch, I could have written that. 
I should write down who on my global is associated with what SG.....
Paragon City Search And Rescue
The Mentor Project
Have you tried requesting in coalition chat if any1 from that sg is online? Try over a period of time and if you don't get any response, you could probably assume the sg is dead.
Thanks everyone for the responses. I currently have three red coalitions on the base entry window. Two of them for over two months now. I won't break the coalitions for a while yet.
And thanks for the coalition chat suggestion. 
Are there any guidelines for maintaining coalition links with seemingly dead SGs? I mean, my SG has coalitions with with other SGs that have the 'can't enter, upkeep due' sign on the base entry window.
Can members of the closed SG still use our teleporters and stuff? Or should I assume the other SG no longer has any active players or they would pay the rent if they were playing?
How long should I wait before breaking the coalition to allow room for other coalitions?