if it's less than 14 days, i don't belive rust occurs.
it's allso not on/off type issue... 14days may equate to an iota of rust that is insignificant, but nonetheless is the starting point of rust.
one thing i know for sure, you can not tell with your eye 99% of the time due to sample size. players go 0-10 all the time. impossible to distinguish wihtout recording and analyzing results.
in extreme cases it may be obvious... take 100 players, leave them in FA until opening day (offseason rust), assign to teams, then watch them all suck for the first month or 2. i bet ~80-90% look terrible in that context, but just a guess.
do that same trial with 14days off actice roster, 28days, 50 days, 100 days etc etc. for a baseline. then re-do with varying lengths of rehab stints... that is the only way to learn how long you'll need.. then extrapolate for points in between the curve or line should become obvious to visualize... if someone has not done this and says tehy "know" ... well...
