Like all things quilting it depends on the quilting design. An all over meander that is not too small or an end to end simple panto can be completed in a few hours. It takes me about an hour just to load the quilt top and backing and batting. This includes center measuring on both back and top and I don't have zippered leaders so I pin to my leaders.
Quilting with lots of starts and stops, custom work or stencil work can take much longer. Doing a lot of background fill can also add to the time and the type of fill it is. A tight meander can go a lot quicker then pebbling. It also can add time if you are doing something new or different then what you are used to. There really is no "average" time because each quilt "calls" for it's own quilting design. Also, like so much in quilting, the more you do of a particular design the quicker you get at it.