They usually wait a year or more for sequels, There was a year and a half (IIRC) between Shakugan No Shana, nearly 2 years I think between Index, Some shows had their sequels faster like Sora no Otoshimono, also SKS3 is like 3 (i think) years after the last sequel, Astarotte no omocha went fairly changed and that manga is also fairly early in the stages but had enough for 12 episodes. It all depends on how they do the chapters actually, if there are only say 6 chapters done, 2 episodes per chapter is nothing new and very easy to do. By the time they start airing the chapters can grow in numbers relatively allowing for more content to be put into the show later on.
Also remember Vampire Bund was aired while the manga was very early and it was pretty much entirely original, but wasn't all that weird. Just not how it should be.