I know it's at least 300 years. Why? Because the grey fox is said to be 300 years old. Now, we know that's not true, the person who is the grey fox just changes when the previous one gets too old. However, in Morrowind, there was no Grey Fox, the theves guild had a typical grandmaster and a guild hall. Meaning that Morrowind took place before there was a Grey Fox, over 300 years ago.

Anybody know exactly how long the gap was?

6 or 7 years...

Really, it was only about 6 or 7 years. The Cyrodiilic Thieves' Guild is very likely a separate organization from the one in Vvardenfell.

According to the UESP, 6 years.

Ninja'd

Yeah its only 6 years. The way I think of the different guilds is that each province has its own guild HQ and each HQ has its own grandmaster. In the case of Morrowind I wouldn't be surprised if both Vvardenfell and the mainland have different HQs for a few guilds.

Morrowind - the year 427 of the Third Era.
Oblivion - the year 433 of the Third Era.

Uriel Septim VII took the throne in 3E 368, and was assassinated in 3E 433. He'd been around since before the first game in the series, which took place in 3E 399.

Freddo is correct.

It's worth noting that TES V: Skyrim departs from the rest of the series by how far into the future its events take place. It's supposedly 200+ years forward since the Oblivion Crisis and the end of the Third Era. That's unprecedented. All of the first four games took place within a span of 34 years.

Wow...I think my friend's Morrowind file is over 3 or 4 in game years! Has anyone passed the 6 year mark? There should be a mod that opens up Oblivion gates somewhere around that time.