YSK: Star Wars: The Last Jedi Tickets go on sale next Monday 10/9
Speaking from experience (I work at a theatre), the first 2 shows in IMAX for The Force Awakens at my theatre sold out in approximately 9 minutes.
Protip: Go to your local theatre to buy them in case the websites crash.
Disclaimer: Don't be a dick and buy a bunch of tickets to resale them for a profit.
Posting from my porn account because I don't wanna get fired from my job lol