How much time would it take to complete the transcription of an average-sized human gene?
From Bionumbers, the average transcription rate of RNA Polymerase II is about 3.8 kb/min. Other researchers measured it as 6.3 nucleotides/sec, or 378 bp/min, a 10-fold decrease. In the same paper, however, they measured the maximum rate at 71.6 nucleotides/sec, or 4.3 kb/min. To figure out the differences between the measurements, I suggest reading the paper, but since it's from Nature Publishing Group you'll have to pay for it, unless you're at a subscribing university or institution (or someone posts the paper in the comments <hint hint>).
So, the answer to your question, like most simple questions in biology, is it depends. It likely depends on the size of the gene, its structure, what transcription factors are necessary for the assembly of the polymerase complex, and, last but not least, exactly how and when you measure. Transcription likely initiates in fits and starts, then gets going faster. This is similar to (but not exactly like) measuring the speed of your car from Point A to Point B. Are you starting from a dead stop, or passing the starting marker at speed? Do you have to start the car first, or is it already running? How many turns and other slowdowns are on the road? Do you stop measuring as the car passes the finish marker, or when it comes to a complete stop? All of these things figure into the final answer, and the result given is entirely dependent on the scientists' choices of methodology.
If you are interested in a broader discussion, I got started on this page at Bionumbers.