How long should I stay at my first job?

Currently working in the AM division of a BB bank...and I already know that I am trying to segway out of finance after working for almost a year now. Not that there is anything wrong with my job, but imo it takes such a long time (~7+ years) to really establish yourself as a strong player at my firm, and it seems to be fairly political once you hit VP. I've been eyeing a few jobs at startups as a business development associate/product strategist and it seems like a better environment for me earlier in my career. I am currently at the interview stage for some companies that have received enough financing so I can not sacrifice too much of my base salary if I were to switch over. Eventually my goal is to work in helping products grow or a marketing role within industry (preferably in consumer tech) post-MBA, which is kind of what I would do in this start up role if I were to transition.

My current firm also is indifferent on people trying to get an MBA (in no way does it accelerate your promotions), but I find it to be something I would like to go through even considering the time and money. I would like to optimize my chances at getting into a top 7 MBA, but I am concerned that if I do not stay long enough in my current role it would be hurt my chances.

My question is, should I leave my current position at this well known firm and work 2~ years at a 200 person startup if I perhaps want to get an MBA later? Or should I draw it out for 3~ years at my current firm before applying? Are top MBAs more willing to accept people with entrepreneurial experiences or more distinguished companies? I just want to maximize my career growth/MBA chances within the next 3-5 years of my life.

Stats if needed: Passed CFA level 2 BBA from large state school with a 3.7 Aiming for a 700 for the GMAT