For my purposes, I consider anything less than about 10 minuntes in bright sun as practical for in-camera. Sure, this isn't anywhere near what you can get from even enlarging paper (which can be much less than a second in full sun), but it's still useful.

Is "cyanotype rex" simply the same formula as the regular cyanotype, but just with the paper coated with citrate first, and then ferricyanide after exposure? Or is there a different chemistry requirement?