In my mind, the standard for foxtrot is to have the footwork, balance, poise, and understanding to move smoothly with drifted timing below 30 mpm.
For leaders this is physically about posture and building the strength to balance forward on the foot, for accurate and sustained feathers and followers heel turns. But its also about building a feel for the "swing from behnd the music" scheme of timing, and that may take the longest to internalize.
For followers, I believe the major gating factor is learning footwork for sustained, slow movement. Specifically, a willingess to take the weight onto the standing heel while the moving foot is still moving. This is so dfficult in court shoes that many dance for years without learning it.
I have recently been finding that students of only limited social background, who are willing to put on chunky heel practce shoes and diligently practice creating movement by sending the body from the standing foot (and related heel turn drills) are making dramatic progress towards a smooth slow foxtrot in a timeframe of 1-2 months.
Foxtrot is a difficult dance, but I suspect the reason it usually takes so much longer to learn is not because of extreme difficulty of the actions, but instead difficulty in identifying and steering sustained practical effort precisely to the things that need to be learned.
Common mistakes that set learning back: reaching the moving foot instead of sending the body, trying to interpret SQQ literally, trying to turn with weight on both heels.
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