Gut-estimate: 60 days, with a 95% chance that my gut-estimate would be between 2 and 600 days if I thought about/investigated this for another day.

[ETA: My thoughts here are very very tentative, and easily changed by a new consideration/datum being pointed out.]

Apparently, the Open Phil meta team was surprised by how flat the distribution of impact they measured was, and the distribution seems to only span 2-3 OOMs (importantly, this rough OOM range isn't for impact per person, which is what you would want, but the total impact of meta-EA interventions that seem like they deserve their own bucket). Though my guess is that unless you have an exceptionally good/obvious personal fit for something, the personal fit consideration leads to 0-1 OOMS, and if this is true, then it is still worth figuring out where the new projects you are considering lie on the impact distribution.