As I figure it. Just seasons, not the stuff that took place before- I'll address that in a couple later chapters
Season 1 (including very first two episodes)- 1 year *
2 wks between (1 year 2 weeks)
Season 2- 6 months (1 year 6 months 2 weeks)
1 year (2 years 6 months 2 weeks)
Season 3- 2 months (2 years 8 months 2 weeks)
3 months (2 years 11 month 2 weeks)
Season 4- 1 month (3 years 2 weeks)
6 months (3 years 6 months 2 weeks)
Season 5- 1.5 months (3 years 8 months)
9 months (4 years 5 months)
Season 6 technically one day (entire season 2 months)
6 months (4 years 11 months)
Day of the Departed- two days
2 weeks (4 years 11 months 2 weeks)
Season 7 - 2 weeks (5 years)
One year (6 years)
Season 8- 1 month (6 years 1 month)
1 week (6 years 1 month 1 weeks)
Season 9- 1 month (6 years 2 months 1 week)
1 month (6 years 3 months 1 week)
Season 10- 3 days
9 months (7 years 1 week)
Season 11** - 1 month (7 years 1 month 1 week)
9 months (7 years 10 months 1 week)
Season 12- 2 weeks (7 years 10 months 3 weeks)
*I had S1 shorter but the comics (admittedly I'm not sure how cannon they are) say the ninja had been fighting the snakes for a year before Lloyd is grown so I changed it.
** Time between realms doesn't always work the same, but since there's not a ton of reference to Wu and PIXAL during the Ice chapter I kinda 'winged' it
Other definite markers: One year between S8, one week between 8 and 9, mentions of days passing in various seasons
Relative markers: ninja are still cleaning up at the beginning of seasons 2 and 7 from the seasons before, there is a lot of technology in season 3 which takes time to build, Rumi is little in season 2 (10-12 y/o) but has to be at least 17 by season 8. Lloyd's voice changing in S8.
Point of contention- season 9 Kai says they're still teenagers... ehhh doesn't really work very well: would have to have been 15/16 when Wu recruited which is feasible but even my modest timeline puts them at 19ish but that would put Nya as under 18 going into season 10... So I'm going to kinda of ignore that statement. That also makes it difficult when one realizes there's several references to The Serpentine War and the battle against the Time Twins being 40 years earlier- Maya and Ray would have had to have been in their late 30s early 40s when they had Kai and Nya. We know Misako didn't marry Garmadon until after the war had at least started but that leaves a very small time frame from which Lloyd was at least conceived to when Garmadon went full evil, tried to take the Golden Weapons and was banished by Wu. Even less time when the show hints Lloyd and Garmadon had some form of familiarity in S1 when Garmadon comes to save Lloyd from the snakes. Then there's the question of why does Wu and Garmadon look like young men in the clips from the Serpentine War but much older in "present day" whereas Maya and Ray still look the same? (Granted both Wu and Garmadon are well over 100 canonically so maybe their age just caught up with them quicker)
Sooooo all that to say the timeline is wonky. It's possible the inhabitants of Ninjago don't age on the same timetable as the viewers (likely).
For my purposes though I'm putting Cole and Zane at 19 (though Zane's debatable depending on when he was built) , Kai 18, Jay 17, Nya 16, Lloyd 11 at the start of S1.
