Just to throw a random line out there, I say anyone with a champed OML is a veteran (or anyone that had one and sold it when he was nerfed).
I’m not serious, obviously there are people without a champed OML that qualify, but when I see a roster with some champed 5’s, but only 1 or 2 covers on their OML and Surfer I consider them a newbie. 
I see your damage to mutants boost and raise you Sentrybombing
My two cents: I think that a genuine vet is someone who has played significantly in every tier, who has a good understanding of all of the major game dynamics including PvP collaboration, MMR, PvE strategies etc.
Realistically this is almost certainly going to be someone who is playing top-end/has played top-end 5* PvP and can play any CL in PvE.
This generally ends up being players who’ve passed 4-5 years, and by default it kind of excludes more recent whales/folk who bypass tiers (why would you anyway?).
Having over 2000 days played…
I think if you have 2 years, you are a vet. You have seen the anniversary come around twice at that point. making it 2000 is like making a whole new tier: Methuselah level
I’d like to consider myself a vet. That does not equate to an “expert”. I spent my first 3+ years playing haphazardly (with no focus on progession, placement, resource hoarding) in a casual friendly alliance. I just built my roster evenly (no concentrating resources on meta characters) and played with no schedule whenever I felt like it. Eventually found my way into a top alliance family who showed me the “ropes” and took my game to another level in 2 years of optimal play, strategic roster building and resource hoarding. Any to this day, I’m still learning from others who pay attention to in game details and share in our alliance chats. I can’t believe it’ll be close to 6 years next MPQ anniversary.
There is obviously a lot of variation in how people use the terms, but I think in general people say “whale” to mean “people who have spent a significant amount of money on the game” and “veteran” to mean “people who have a played the game enough to have a decent understanding of all of its various systems and meta-systems.”
Those things are all subjective of course, so your “substantial amount of money” or “decent understanding” might not be the same as mine.
I think it’s hard to be a “vet” at all if you are still in the 1*-3* space (and reached 3* land after ~ March 2015) because the 4*/5* endgame has so many unique quirks, both in gameplay and meta-systems. But 4* players, especially those with huge 4* rosters that choose to soft cap, can see most of what the game has to offer if they want, so I am sure that there are lots of such players that would qualify as “vets.”
It’s also important to remember that no one person really has a full understanding of the entire game experience anymore. I started in 2014, and haven’t been HP/roster slot limited since 2015. It would be pretty hard for me to claim much meaningful experience with the current state of play for 1*-3* rosters, even though I worked my roster up the old-fashioned way, with 2:24 refresh timers (wasn’t it some odd length like that?), unlimited shielding, and only 5 healthpacks (plus it was uphill, through the snow both ways!).
Similarly, the uber-whales and level 525+ crowd can’t really claim much meaningful experience with life as a mid-iter 5* player like me, let alone a baby seal. . .
@DAZ0273 your post inspired me, so I came up with a list that may be too extensive to share here, so here’s a shameless plug!
https://forums.505go.com/discussion/79597/you-might-be-a-veteran-player-if/p1?new=1
In my opinion most veterans have 2* and 3* farm, lots of 4* champs and at least one cover in each 5*. And they are willing and capable to advice other, newer players.
And avoids “Meat” Rocket and Groot…
A true veteran is someone for whom MPQ has existed for less than half the time that they have been playing various incarnations of puzzle quest.