Ubers isn't part of the tier shift every three months, so there is no specific point where a Pokemon can be considered broken in OU. Ubers doesn't really work well with the system since the dynamic of the metagame is very different to others. For example, Greninja was deemed to good for OU in a suspect test, but it would never have made it to Ubers through usage because it is nearly useless there. Likewise, there are plenty of Uber Pokemon that don't meet the 3.41% cutoff in their tier but if they moved to OU, they'd be completely broken.
In every other tier, 3.41% in the tier above is the benchmark. Most Pokemon in the Ubers tier have been there from the very beginning (for example, Xerneas and Yveltal were never OU at all), or were suspect tested in OU and kicked to Ubers (Blaziken, Greninja, etc.)
There's a flaw in the logic that a simple usage stat in OU can determine how "broken" it is, because the fact a broken Pokemon is available doesn't necessarily mean people will use it. Greninja was a number one in the month before it was banned (26.7%), but other examples of banned Pokemon do not follow suit. Mega Mawile was at #22 with 9.6% before it got the boot, and Landorus was at #19 with 10.1% before it went to Ubers. So there's no accurate measurement of how broken a Pokemon is, because there isn't a close enough relationship between usage and what Smogon chooses to suspect test.
