From what I hear, most Phd programs take 5-6 years on average. I know this seems like a lot but that's after getting a bachelors, not after a masters.
On a side note, many schools I've seen don't offer a masters degree in physics, but you can sometimes get something called a "masters in passing" while you are doing Phd, where they give you a masters because you completed the equivalent amount of work for a masters on your way to a Phd.
I don't know anything about the difficulty but I know a little bit about the competition and grading (at Berkeley anyways). Grading curves for undergraduate courses tend to be a bit harsher than graduate courses and the students tend to be less competitive from what I hear (though competition in undergraduate physics courses in upper division courses is nonexistent in my experience).
I don't know anything about the differences between universities but what eXorikos said makes sense to me. I imagine where you do it also may help getting a job afterwards, since a more renowned university may lead to more renowned references and connections.
As for getting paid, when you are a grad student, the university tends to give you a "job" of some kind. It is either a TA (Teaching Assistant) or RA (Research Assistant) job from what I hear. TA jobs help out with classes and are given by the university. From what I understand RA jobs are funded by research grants and are given out at the discretion of faculty members. Typically the pay is supposed to be all your tuition and fees and a stipend to help make ends meet. The size of the stipends depends on who you are and who you are working for from what I hear. I know a guy who got into a Phd program at UCLA and according to him, he would get expelled if he got a job because of the amount of money they are giving him. I don't know if that includes full time and part time jobs, excludes jobs at the university, or any other possible technicalities.
I don't know anything about Cambridge but I imagine it would be difficult to get into.
This is all from my own personal research and not from experience. I could be wrong but this is what I understand to be true.