Nah, 6 probably didn't get as long. For all we know between saying goodbye to rose and returning a few seconds later at the end of Rose, the Doctor could have travelled for years!
And again for all we know 8 might have had the longest life of all because we have no concept of how old he was when he regenerated into 9.
Pretty much yes.
Like when Doc 4 left in the Tardis in "Robot" (The nook version of "Face of Evil" said this was when he landed on the planet the Seveteam and Tesh lived on, and then went back to Earth), or when he had to leave Sarah, who knows how long before he went to Galifrey in "The Deadly Assassin", and then in between "The Deadly Assassin" and "The Face of Evil", could have been centuries. And we don't know how long the Doctor and Romanna travelled togather, or how long the Doctor and Adric travelled togather.
Same with Doc 5...we don't know how long Trakens live....Nyssa could have travled with the Doctor for 20 years and looking like she did not age a day.
The 9th Doc (Whom I prefer over 10), could have been away for a long time before going back to Rose in "Rose".
Doc, after leaving Rose on Earth after meeting her: Ok, time to move on.
~Several decades or even centuries pass~
Doctor: Hmmm....say, I think I'll head back to Earth and see if that Rose girl still wants to travel with me, I'll land about 30 seconds after I originally left.
Plus we don't know how long time went by when #10 left Donna for the first time, or from "Voyage of the Damned" and "Partners in Crime". Same with the time between "Journey's End" and "The Next Doctor", and the story after that one.
Lots of ideas there.