The Dragon Priests in Skyrim all have similar looks and powers, were they all in power at the same time or were they some form of lineage spread over hundreds of years?
Hard to tell: they've been around for thousands of years, and can stay alive for thousands of years
The in-game book The Dragon War tells us that that worship of dragons began in the Merethic Era (i.e. before the First Age, which began about 4700 years before Skyrim*). The worship was brought by Ysgramor around ME 1000 (i.e. 1000 years before the First Era), who was the chieftain that brought his people to Tamriel, the descendants of whom are modern day Nords.
In the Merethic Era, when Ysgramor first set foot on Tamriel, his people brought with them a faith that worshipped animal gods. Certain scholars believe these primitive people actually worshipped the divines as we know them, just in the form of these totem animals. They deified the hawk, wolf, snake, moth, owl, whale, bear, fox, and the dragon. Every now and then you can stumble across the broken stone totems in the farther reaches of Skyrim.
Foremost among all animals was the dragon. In the ancient nordic tongue it was drah-gkon. Occasionally the term dov-rha is used, but the language or derivation of that is not known. Using either name was forbidden to all except the dragon priests. Grand temples were built to honor the dragons and appease them. Many of them survive today as ancient ruins haunted by draugr and undead dragon priests.
So if this writing is accurate, then "dragon priests" were around for thousands of years.
As for "contemporaneous", that's a bit tricky because Amongst the Draugr indicates that the dragon priests are given unnaturally long life by siphoning off life force energy away from their servants.
When the next group of draugr came to pay homage to the priest, I noted a sort of transferal [sic] happening. A distinct flow of life force between the adherents and the master.
It was here that I finally understood the dragon cult's notion of resurrection. The second eternal life was only promised to those who ascended to the priesthood, but the lesser functionaries contributed their life force to sustaining them for eternity. I don't know what sort of eternal wellspring they draw from, but it's clear that each draugr carries only the barest whisper of life in it, and rekindles it nightly while resting in its niche. I now believe that the grotesque forms that we see in the barrows were, in fact, buried fully as men and women, and only over the thousands of years that have passed withered into the wretched things we know. If we had visited a barrow directly after its construction, we might not have even known any of its inhabitants were dead!
The last known holdouts of living Dragon Cultists were recorded in Skorm Snow-Strider's Journal where he notes that they were thought to be extinct. This was written in the First Era 139 (about 4600 years before Skyrim):
27th of Sun's Dusk 1E139
It sounds impossible, but we appeared to have stumbled upon a massive hold out of the Dragon Cultists, who were believed to be wiped out during the Dragon War. The Elves must wait, as this is a threat we cannot ignore. If we are quick, we may be able to catch them unaware and avoid a lengthy siege.
So about 1140 years passed between Ysgramor's arrival and this last known holdout of Dragon Cultists. Given the wide span, I'm inclined to think that there were multiple generations of dragon priests within this millennia (rather than them all being from the same generation), but of course they were all kept alive for thousands of years by their unnatural magic.
* For the sake of simplicity, I'm going to assume that 4700 years passed, since that matches the timeline on UESP.net. The reason it's messy is because there were several Dragon Breaks where time ceased to progress in a linear fashion. One such Dragon Break happened in the First Era and created a "timeless time" where there are no written records, and then the next most recent records are dated over one thousand years later. Some in-universe scholars claim that such an event didn't happen and the gap was just an excuse to cover up poor record keeping.
From the game itself:
In days of old, when dragons ruled the earth, there lived mortal men who worshipped the beasts as gods. These dragon priests are long since dead... but something has awakened them from their ancient slumber.
They were the foundation of the Dragon Cult, which ruled along with Alduin.
A lot of information can be found by reading the books within the game.
These are worth checking: The Dragon War
As far as I can tell, the men themselves probably joined the cult at different times, but with the powers granted to them by the Dragons, managed to live extended lives and in a few cases even cheat death.
So it's both a lineage and and in power at the same time. Although, how you move up the ranks when the top guys are immortal liches, I'm not sure.