I certainly would not be amused by any such regulation. Once again, it seems that people who would never drive an hour in a taxi driver's seat, are trying to tell them how to do their job without knowing much - if anything - of what it's really like.
Most taxi drivers I know, work either six, eight, or twelve hour shifts. The few that own their taxis might work either twelve or sixteen hour shifts.
In any case, every driver has his or her own pace, and would tend to take breaks every three to four hours, lasting between 15 and 30 minutes.
Depending on where and when they are working, the timing of their breaks is somewhat flexible; but at the same time, also somewhat predictable. The driver guesses when to take a break, and how long to make that break, by estimating how much it would cost to take a break at that time, in that area.
Yes, it's possible, that, on an unusually busy shift, they might go as long as six hours without a break, but I've never been able to go without at least a quick bathroom break every four hours. While I'm there, I might as well buy something to eat or drink, maybe something to read, maybe make a quick phone call ... Some drivers might buy a pack of smokes, a lottery ticket. Could take as little as five minutes, certainly not more than fifteen.
The ones that confuse me a bit are the ones that take their meal breaks without leaving their car, perhaps only dismounting to use the facilities (it's easy to suspect that a small minority may not even get out of the car to do this

).
If I were forced to legislate anything regarding breaks, I'd set the number at four, knowing that the vast majority of taxi drivers would probably have already taken one before it was required. 'Laws' should not be difficult to comply with for the majority of the population set involved.
I'm much more concerned with the fact that some drivers work more than two twelve hour shifts consecutively.
The last time I made sweet love to a woman for more than twelve hours, I remember taking three breaks an hour long apiece, I didn't do it the next day, and the day after that, I was useless after three hours for the rest of the day.
Most taxi drivers that do shifts of twelve hours or more, are very likely to do so for at least three consecutive days, some even as many as six in a row. I wouldn't feel safe pushing myself that hard for that long, that often.
I say the above as a taxi driver who regularly drove three six hour shifts, and one eight hour shift a week (Sat), during which I'd cover between 20 and 30 miles per hour. Me taking three days off per week was my way of staying sane, sober, and safe.
I really don't know how anyone would want to drive more than twelve hours a day, or more than five days a week. The real problem is when such long workdays and workweeks are seen as necessary.