There are a lot of factors that dictate this such as: Personal attack stat Personal loadout of items Which boss you are attacking The level of the boss you are attacking (as level increases so does the boss' health) Are you using Power Attacks all the time? Do you have friends helping you kill the boss? So in a nutshell there really isn't an easy way to determine this, but if you soloed a Level 1 boss and counted how much Stamina you used, all things remaining equal save for the Boss Level which increments automatically and it would be similar. If you didn't get stronger via items or stats you would likely end up spending a little bit more stamina for each Boss Level increase.
