An overly generalized answer would be they are in Class A for the first few watts typically. Exactly how many watts depends upon the biasing and design particulars. Very few manufacturers explicitly state how many watts you get in class A operation for their amps.
The upshot is, unless you listen loudly, the first few watts is usually all you are using anyway. This is one of the major arguments *against* a pure class A amp. For most users there is not a significant difference. It's seen by some people as merely more wasteful without tangible benefit for most use scenarios. It depends upon your speakers too of course, amongst other factors.
This isn't my personal opinion btw. I'd setup a system around a low power pure class A amp in a heartbeat.