My group has been woundering about this lately too. My advice to the DM was "the Leadership feat says that it is in the DMG because it should be up to the DM to allow leadership or not in his game. Having a cohort or two can make a big difference in a game and shouldn't be allowed across the board. However, if the DM decides to design a campaign where the players are, perhaps, military leaders, then perhaps having a chain of cohorts would be ok.
Basically, if the DM wants to deal with it and he thinks it is suited for his game, then sure. Otherwise, no.
I did a little math and figured that a 12th level character with an 18 Cha. who had a cohort with Leardership and also had an 18 Cha., and his cohort also had leadership and an 18 Cha. and so on, alll the way down to a 5th level cohort, that 12th level character would have a little more than 100 followers and cohorts.
On one hand that's not unreasonable, I can imagine a 12th level warlord with at least that many followers. However, I doubt many DMs would allow so many followers in their typical campaign unless that's what they were designing the game about in the first place.
so basically, my advice is leave it up to the DM. I don't know of any clear rule about it, but there might be one somewhere.