i can't achieve full retraction of scapula which means i cannot do barbell rows,inverted rows etc properly. I think this is due to me neglecting back workouts and doing bench press and military press constantly meaning i have tight pecs, how long would it take to loosen my pecs and achieve full range of motion again if i perform pec doorway stretches 3x a day and rest from gym?
A combo of exercise, soft tissue work, and mobility drills would be your best option. I don't think rest from the gym is a good option. Instead, switch your focus to posterior musculature, go for a 3:1 or even 4:1 pull to push ratio.
Also incorporate some scapular based corrective exercise in your programming, include proper mobility and stretching drills, and look into some aggressive soft tissue work (ART, Graston, etc). If you don't have the money or time or just don't want to go the aggressive soft tissue route, make sure to get your foam rolling/SMFR techniques in.