UPDATE
As promised, I did a test today on the length of time that the WL-20 pair lasts on one charge, and I am happy to report that I was very, very wrong.
I played for about an hour or so, then left the guitar on a stand, with the transmitter unit in it, and also left the amp with the receiver part of the unit in there.
Every so often I would pick up the guitar for ten minutes or so, and I got about 7 hours out of the units before I plugged them in to charge again (and the transmitter was fine when I did that - i.e., it still had a lot of charge; the lights on the receiver, though, were telling me that it needed to be charged, but it was still happily receiving signal when I pulled the plug).
When I wrongly reported previously that the unit was only holding enough charge for an hour's playing, I think that I might have been going on a mixture of two things: first, was others' reports, and second was the fact that I was using the unit wrongly for the first few weeks I had it.
I was not plugging one of the unit's jack plugs into the other in order to activate them/pair them for 'action'. That process usually takes about 14 seconds, but if you do not do it, you get all kinds of pops and crackles, and that is why I thought that the unit needed to be recharged after a relatively short period of use during those first few weeks I had it..
Now, while this is good news, I do occasionally hear pops when the units are fully charged.
Apologies for any confusion I may have caused about this unit.