I've done a lot of md and high heat cooks (350*-550*).  I can get it up to temp and heat soaked in about 45 minutes. I do this type of cook almost nightly.  
I've done a couple cooks in the 250* range. To get it stable and properly heat soaked at that temperature it took 60-75 minutes.  
The one cook I did at less than 225* (average 212* to be more precise),  I took it up really slow and was stable and heat soaked at around the 2 hour mark.  
Personally I like Low-n-slow in the 250*-275* range. Less Startup time, less cook time, stays in the stall less time and it tastes the same in the end. 
 YMMV.