How far do these fields extend in a measureable sense?
That's a hard question, because I don't know what you're measuring with, and for how long!
But: if properly designed, these are (luckily) not radio emitters, but just the primary side of a transformer. Magnetic flux (what's measured in Tesla) drops with the square of distance, but what really matters is that Gauss's Law for magnetism says that the integral over any closed surface, for example the one enclosing the charger with phone in operation, is zero.
That really means that no flux escapes – just like you putting a permanent magnet on your fridge doesn't have any electrical effects on your phone (unless you move any part of this).
Energy could be radiated if the sizes involved would allow for a perpendicular E-Field to form. They don't, usually – but as Elliot said, don't trust anything to be perfect. Especially, the switch-mode controllers for these transformers won't produce perfect cosines; expect harmonics that also couple into cabling and if high enough radiate. Again, up to the sensitivity of your measurement, whether you notice. But: these are exactly the same problems that any SMPS has, as Elliot pointed out!