How far in advance should we purchase tickets for the high speed train from Copenhagen to Stockholm?
They go on sale up to 6 months in advance of travel at www.sj.se. You'll notice a big difference between fare choices: "Non-rebookable" (cheapest, and often sell out in advance), "Rebookable" (these are the rates shown on Rick's train fare map), and "Refundable" (much higher fare, but unrestricted). There's no deadline to purchase, though a particular departure could fill up.
Tickets go on sale four times per year, but buying them one or two months in advance is usually fine. Prices start at 195 kr one way and go up the more popular the departure becomes.
As Badger said, SJ releases their tickets in batches (unlike some other railway companies - like the Danish DSB, that continuously release tickets X amount of months in advance). SJ tickets until June 21, 2020, went on sale back in November. SJ is planning to release tickets for June 22, 2020 - August 2020 in the middle of March.
Track works can sometimes "delay" the release of tickets on days with track works that require alterations to the timetable.
Thank you everyone, this is incredibly helpful!
The naming of the ticket types are confusing though. Rebookable are pretty useless since you only get a voucher for the money you paid for the ticket. So if you need to rebook to a departure close in time you probably need to add a lot of money since prices are usually high close to departure. Refundable are in addition to refundable also possible to rebook (maybe only online or in the app) for free to another departure the same day, but you can only perform the rebooking on the travel day or the day before.
Thank you, Anders. This is very helpful!