It's random, much like in a casino or in the lottery. While opening more increases the chance you will get the prize, it is not guaranteed.

A friend of mine, pure f2p got tendo inside the boxes. He was lvl 77 or 79 at the time.Personally i have never gotten any ninja and now of only one other player who ever fot a ninja (sasori 100) in our server since it was opened. Then again, one server is not a sample that reflects anything statistics wise.

I really believe its rng. The chance of getting a Ninja though is probably around 0.1% so you would need around 3000 opened boxes to have *most certain probability of getting a ninja.

On an off note, tobirama is probably the weakest of the premium ninja available so i would recommend to avoid spending for him and save your ingots/coupons for tsunade, hashirama or DB hidan.