Well, Bottle A is the old sized Tamiya paint jar - 23ml. Bottle B is the new size - 10ml.
The extent of use is dependant on how much you thin the paint to begin with. Example, I thin my Tamiya paints up to 75% - increasing the volume accordingly. I can get about three 1:35th scale models done with this volume typically - using the old 23ml size.
The smaller bottle, thinned the same, lasts me about two decent sized 1:35th scale models - or three 1:35th scale trucks...
Your mileage may vary.

Sorry, I'm not a user of Humbrol Paints, so I can't share experience there...
Gunnie