If Sauron prevailed...

[Zigûr]

This topic was discussed here:
http://forum.barrowdowns.com/showthread.php?t=18770
My opinion is that Sauron's realm would have eventually torn itself apart through internal strife and civil war. A major theme of Professor Tolkien's work is the fissiparous nature of evil and I believe that thematic consistency would dictate that Sauron would, in time, lose control of parts of his own empire. Ring or no Ring, Sauron's will and attention was not omniscient or omnipresent, and Cirith Ungol alone shows how easily his own servants could fall to fighting each other. This was especially a problem with Orcs, as Professor Tolkien observes in the essay 'Orcs' published in Morgoth's Ring.

All his Orcs hated one another, and must be kept ever at war with some 'enemy' to prevent them from slaying one another.

While Men, not Orcs, would probably be the majority of Sauron's population, we might imagine Men becoming Orc-like under Sauron's repressive, degrading regime and developing similar traits.

Oppression breeds rebellion as well.

That being said, such a process might be expected to take hundreds, if not thousands, of years.