Who knows? There are so many factors, many of them unpredictable or unknowable to guess.
Looking from the chart:
It took about 2.5 years to go from 100M ISK to 150M ISK the first time, so that’s a good a starting guess as any. Probably it depends on how many were dropped during that last event, but judging from how little it has moved so far, I think it will take a while, although 2.5 years seems a little long to me.
I’ll go then with 9-12 months as my guess then. Speculators aren’t likely to try to push up the price having been burned once by CCP, so it will take however long it takes for the excess to be consumed, but even then there is no guarantee CCP won’t add them to the loot table of another event and push the price right back down.