Many replies, which did not have time to read, so I will simply answer from my own experience:
There is no set limit in years between each incarnation, even the idea is absurd as time do not exist on the spiritual planes, and a person can basically be reborn as soon as he dies. However normally a person do not get reincarnated that soon but spend some time in the lower astral planes for a while, some ascend further up, and some are never reincarnated. Some only reincarnate a part of themselves, a soul fragment.
For further information on the last part, please check my thread at
http://www.projectavalon.net/forum/s...ead.php?t=6337 where I explain this concept in depth and I think you may find it quite interesting in regards of your research.
I also explain there how several people claiming to be the reincarnation of the same past person can in fact all be correct about it.
Peace profound,
-EA