Of course it's subjective, and there's no "magical number", it may not be clear, but there is a line between regular "progression" and a straight up "grind", in the last few patches they slowed down progression 4 times (both by lowering exp gain and increasing exp requirements), while also introducing plane "upgrades" which are neither realistic, nor historically accurate, nor do they benefit the game play in any way, they don't add any depth to the game or modification system, those artificially stretch the game by forcing you to use and underpowered, slow airplane each time you unlock one and, hopefully (for them) you'll get frustrated enough to buy yourself a way out (compare a "stock" 109F4 to an "upgraded" one, for example, or a P-47)
To me, playing a good match and seeing that the "progression" bar has moved by less than 1mm, is enough to realise what they're trying to do, spending hours to unlock 1 airplane + earning "credits" to "train" the crew for it + credits for additional skill training + credits for upgrades +.... You can see where this is going.
Where is the line between XP boosts for premium players, and an 'extended grind, just so people buy premium'? I don't see it.
Again, It's subjective, but the "extend grind, buy premium" starts when progress without "boosts" is unreasonable, so yeah, the line is somewhere between "I can buy premium to save up some time" and "I need to buy premium If I don't want to grind for 7 hours for next plane", yes i know they kinds sound the same, but you know what I mean, If premium account is the only way to have a reasonable progression, they you are past that line (and, to be honest, some games even with "premium" or whatever, are horrible grinds, because 1.5x0 is still 0)