I just did a flush and fill on the radiator for my 99 gsr. I started up my car and let it run for about 20mins( got tired of waiting) and fan doesn't come on. Should I let it idle longer , say 30mins. or so? Open to any advice.

 

I think that it's based on temperature not time. There is no load on the engine at idle so it will take a long time to warm up. I've heard this is not good as the coolant doesn't circulate as much as when you are out driving, that could just be a myth tho. Also judging from your location it was pretty cold out so you may not need the fan after only 20 min of running with fresh coolant.