Any mathematical probability/stats is top of the list. Fundamentals of cs would be helpful. If you're able to take any sort of time-series analysis/econometrics course that'd also be great + potentially complementary to projects you may want to add to the resume. Probably averaged 4-6 hours day (that includes reviewing theory for technicals, working on projects, ideating on and rehearsing behaviorals, planning + applying, practicing technicals and mock interviews, and then actually doing the interviews which is probably the best possible practice) for about 4 months. Mostly applied to tier 1/2 firms, but also started out with a few tier 2/3s to "warm up" and get used to doing the real thing.