From what I've observed from playing for roughly year so far now, is that it's roughly a year for around 1300 - 1500~ or so not including free draws & taking advantage of the Rebate by saving 300 Seal Scrolls, Edo Hashirama is anywhere between 2500-3000 pulls.

So 2-2.5 years of playing just to pull the Ninja, assuming you save your coupons you should be able to ac*ulate enough through out those year to buy both Tobirama & Hiruzen, it's possible- you would just need to be insanely dedicated for a very long time lol.

It's why a lot of people will call a lot of random things "Free to play", technically the only thing gated in this game is Ninja Medals.