Well, for my comic it kind of changes depending on the strip.
Depending on the quality of the strip, like backgrounds or number of panels, it may change. My longest time I think was 7-8 hours, but more recently, my best time has been clocked in at about 3 and half hours. But I don't color, only do black and white with greyscale shading, so I wouldn't really know proper times compared to everyone else.