The criteria of time over number of fights isn't arbitrary. You improve in the gym and with physical maturation. Cerrone ripped off 7 fights in just over a year. At the end of it, when he jumped back up in competition, he had 3 straight decisions, the last of which he should have lost. The reality is he's not much different of a guy than he used to be.
Is the time frame I chose to constitute "recent" arbitrary? It's hard to measure. But something like 4 years is a long time in MMA, especially when you're 23-24 at the beginning of it, and were making your octagon debut. Pettis has only had 4 fights in 3-4 years, but has won them all dominantly, against elite comp, in ways other fighters weren't beating them. That means he's creating separation w/ the division and is special.
You think Pettis is unproven as a top fighter, what does that make a guy like Khabib? One legit good UFC win, a more or less controlling decision over RDA. Then he should have lost to Tibau a year more recent than Pettis officially lost. This is the #2 fighter in the division. RDA lost to him. Pettis dominantly beat the other 3 of the top 5 contenders.
I think it's fair what I said. The hopes at lightweight for other special fighters come from move ups via fw like Aldo, maybe McGregor, and maybe Edgar; as well as prospects like Magomedov and Felder.