Honestly, I don't know because I don't do single miniatures. The more miniatures you work with at once, the less time it takes per miniature. I can't finish a halberdier in one or two hours, but I could probably finish 5 halberdiers in 5 hours. I can easily finish 20 halberdiers in 20 hours.
Another thing that skews my records is the fact that I have quit gaming so I have very little need to paint miniatures fast, let alone assemble and paint whole regiments in one go. I might prepare a handful of random miniatures one day, glue sand on the bases the next day, wait overnight and touch up the bases a bit, wait 7 months and before undercoating them and finally start painting them 2 years later, just to lay them down the next day and buy, assemble or paint something else. Rinse and repeat.
If I were painting a new army for say, a tournament, it would still take about two days at minimum after opening a box to even undercoat the miniatures. Plastics are total pain in the a** if you need to finish something quickly, metal models are so much easier. My deep hatred towards assembling plastic regiments means that I almost never even try to make models from box to finished as fast possible.