According to USB Specificcation - revision 2 :
that can be found here:
"4.3.1 Power Distribution
Each USB segment provides a limited amount of power over the cable. The host supplies power for use by
USB devices that are directly connected. In addition, any USB device may have its own power supply.
USB devices that rely totally on power from the cable are called bus-powered devices. In contrast, those
that have an alternate source of power are called self-powered devices. A hub also supplies power for its
connected USB devices."
Chapter 7 details the electrical part.
The rule of thumb looks to be a minimum of 750 mA (5 units x 150 mA - 750 mA) must be available for all USB 2.0 standard output. But this is only a part of the story.Some hobby users report a 900 mA by using "Battery Doctor".
Digging deeper into the problem, I found that some phone manufacturers limits the maximum charging current to some 100-150 mA in PC case. Whether this is true or not, I do not know. But it is in line with my findings that (all) my phones charges from USB PC in far longer time than in the case of wall socket adapter.
Another point that I found interesting is that there is very little info about total available current to all USB ports alltogether in a PC. I bet it is somewhere more info, but it takes to long for me and it is much more complicated than assumed at a first sight.
In my case, there are 8 ports, out of them I use 4-5 at a time, 1-3 incidentally and 3 of them are available for my desk work - including powering the Arduino Board while coding and prototyping.
However, maximum current rate is, well - maximum.
So the problem is rather: what is the current value that is available from a regular PC USB2.0 under real life circumstances, and for a long time?
Now, for the motor side of the story, the relation between mechanical power, nominal voltage, current and efficiency on-spot and long time is something to study and - in my view- to check.
I found something more intuitive here:
What I would do - and I will do when the time to my motors will come - is to connect a laboratory power supply to the motor and measure the current by drawing a graphic of current/time (at various voltage) - to account for the coils higher temperature - I suppose it will rise, but I shall see.
Then there is the point of starting the motor. I do not know if I can measure something at that moment by a digital multimeter. I am not experienced enough to use an oscilloscope in this purpose, but I will give it a try.
Maybe these rows are not directly related to the topic, but I hope it helps, at least for the part of rising questions - that I admire in every human and I think it must be encouraged.
De omnibus dubitandum