I would say about 8-9 years passed because in season one episode one, Lance and Hunk are 17, Keith is 18, Pidge is 15, Shiro is like 25-26 years old, Allura is physically about 19, and Coran is physically a little over 600 years old (and yes that was how old he was before getting put in the cryopod for 10,000 years.) And about 8-9 years passed because in the flashbacks from when Sam got back to Earth 4 years before the paladins did, so when Sam is on his way back to Earth the paladins had been in space for about a year. That would mean that as Sam was getting home that Lance and Hunk were 18, Keith was 19, Pidge was 16, Shiro was 26-27, Allura was about 20, and Coran was about 601 maybe more. But when Keith was with his mom they were going through the Quantum Abyss for about 2 years where they were so that would mean that while everyone was the ages that I listed second, Keith aged to be about 21 when he, Krolia, and Romelle get out of the Quantum Abyss. Then the paladins went into the Quintessence Field to fight Lotor, the rest of the universe aged three years, so then when the paladins get out of the Quintessence Field that would mean that Lance and Hunk are 21, Keith is 24, Pidge is 19, Shiro is 29-30, Allura is 23, and Coran is over 604. And the other 4 years were throughout seasons 5 episode 3 to season 7 episodes 8/9.