It is always about relative position and power. Take North American Free Trade Agreement for example. The original deal lasted 25 years but from the get-go the Yankees never abided by the agreement. One major component is the soft wood lumber agreement under NAFTA. Mind you: it started in Bill Clinton and Jean Chretien era - a very harmonious period between the US and Canada. Every 2 to 3 years, the Yankees would impose punitive tariffs on Canadian lumber and the Canadian government would challenge it under NAFTA and WTO rules and win. It is like clock work and it never fails. And it is still onging. If you don't believe me, ask Cyber Horse or someone from Canada to confirm it. 

THe point is that with respect to the US Canada is in a very weak position it has no choice but to live with the agreement. Trump has the power to tear up the agreement and did exactly that. In China's case, for now China still has not completed the economic reforms and needs to bide her time but in 7 or 8 years time China may not need to at all. 

Any trade deal lasts from 5 to 10 years and more or less preserving the status quo would serve China's interests very well. After that, the ball is really in China's court. If I were in charge, I would not rule out China extracting her pound of flesh.