It really depends on the situation. I've only had two protectorates up till now. One were the Arverni, and the turn after the agreement was signed it gave me 2,000,000 income from tribute They never broke it, I did instead after 50 years or so to wipe them out. The other time as the Casse I got the Aedui on their knees, but they gave me zilch money and broke it quickly.