A five year arc which wasn't really completed, considering a lot of it was rushed in Season 4 and then seriously watered down in Season 5 until the last few episodes, which left countless threads open.
I'm trying to remember the source here but drawing a blank, IIRC B5 was mapped out entirely in advance but in the end got cut short by at least a season.
I don't recall him ever saying it would have more than a 5 year run.
I'm pretty sure I never saw anything on Lurker's Guide while the show was airing. By seaon 3 JMS had definitely decided on a 5 year arc (He constantly mentioned this fact) so if there was ever a 7 year arc planned it was an idea abandoned earlier.
When making the 4th season the network was slow about telling JMS that he would get a fifth. That in essence cut the show short since he rushed the story to finsh in time only to later be told he had got a 5th season.
That said JMS pitched ideas for DS9 to Paramount which did have a seven year run. When they said no JMS decided he didn't need the trek franchise and made the show himself. Could that be the source of the confusion?
Incidentally that's not the only time the Trek producers turned down a good idea in favour of a crappier one. Andromeda for instance was supposed to be set in the Trek universe after the fall of the Federation.
He says 5 year arc alot in the DVD commentaries. He was told season4 should have a conclusive ending (as karajorma says) because a fifth series was far from confirmed at the time.
Well that's what this new movie should help with. Lets hope JMS is back on form. Crusade was good but never had a chance to get going. I wasn't ever that impressed with LotR though.
And the entire thing is going to suffer without G'kar in it. unfortunately.