Question: what season and episode does elena go into a coma?
Answer: 5 or 6 years. Kai puts Elena in the coma in I'll Wed You in the Golden Summertime which takes place June 23, 2013. She does not wake up until just before May 2019, when Bonnie somehow finds a way of breaking the spell. This puts the time that Elena was in the coma at 6 years give or take, as mentioned above.