My understanding of the matter is that after the exclusive period expires after the end of the campaign, that the campaign mythic enters the general mythic pool as is available through normal methods. That includes pulling it from gem and VIP keys, and event keys when the appropriate week for that mythic arrives. In the case of Kurandara, that would be Urskaya’s week.
That said, if I remember right from before Taransworld went permanently offline (someone who has a copy of the schedule can spot check me on this), Urskaya has an event week during the campaign. Which would mean that Kurandara would still be exclusive during that week and need the next the Urskaya event week to occur before event keys could pull it. That’s surely a long, loooooong time away.
One might as well figure at least 6-8 months minimum as a rough estimate, as kingdoms only can have 1-2 event weeks per year (52 weeks / 34 kingdoms = ~1.53 kingdom event weeks per year) assuming a generally normal distribution where everyone gets their turn being the event week kingdom.
IMO, Soulforge or bust if one goes the free route on the campaign.