How long will the Messianic period last?

Belief in the Messiah is one of the 13 principles of faith. However, when the Messiah will come and how long the messianic period will last is the subject of dispute. The gemara (Sanhedrin 99) offers the following suggestions for how long the Messianic period will last:

1. R' Eliezer - 40 years
2. R' Dosa - 400 years
3. Rebbi - 365 years
4. Avimi - 7000 years
5. R' Yehuda - the amount of time from creation until the Messiah comes
6. R' Nachman Bar Yitzchak - the amount of time from the flood until the Messiah comes

Interestingly enough the Gemara earlier stated that the world will last 6000 years, 

Sanhedrin 96B - Six thousand years were decreed upon the world: 2000 years of emptiness (without Torah), 2000 years of Torah, and 2000 years for Mashi'ach. This opinion of 6000 years is generally accepted, however, opinions 4-6 above clearly disagree with this.

The conclusion that we can draw from this is that like with many things Chazal had no clue about how long the messianic period will last and the numbers range from 40 - 7000 quite a large discrepancy. Considering that belief in Messiah is a principle of faith it is astounding that there is such a wide range of opinions. Of course, if Chazal had no clue about the Messiah what else did they have no clue about?