I don't know if anyone knows the answer to that now. It was certainly a lot faster to raise loyalty when there were rioters to arrest and invaders to kill but, even then, in certain towns it still took a considerable amount of time (at least it did for me in Yew)..many of us now are just maintaining. You might have to be the test case to see how long it takes now starting from scratch. The good news is you only have to get to respectable to get a town banner. The most time consuming was trying to get beyond that.
I haven't been motivated enough to try this yet, but I heard that donating lumber is a fast way to raise your city loyalty.
Also, if you're going to raise it by doing champion spawns, make sure you've used the loyalty gump to declare your citizenship with your chosen city first!
I don't know if this is mentioned anywhere else, but I believe it takes a week to switch your loyalty from one city to another, if you're thinking about running for election anywhere. We should see the City Heralds start shouting about nominations soon, as the city stone gumps say that elections begin on the first day of March, June, September, and December.