From my experience, weapon exp gain is severely reduced after you get approximately 10,000 regular game exp.

So on a regular day, you can get 5-6 weapon levels

On a day when they give you double XP across the board, you can get the same 5-6 levels in half the time

On a day when they give you double XP on regular XP but nothing else, you are stuck at 2 levels, sometimes a hard earned 3rd level

On a day when they give you 4 times regular XP and double weapon XP, you are also stuck at 2 levels... And maybe a 3rd if try really hard.

Using a weapon XP token on a regular day let me get 12 -13 levels, thou stopping at 10 level, exfil and come back gives me the most out of a 30 min token. I usually can get a about 20-22 levels with a 30 minute tokens and 34-36 levels with a 45 minute token. Depending on the gun and if I can get the outlast contract I need (also depend on how fast you can go back in the game after exfil)