Not a lot. I'm not sure of where these things usually top out but at least for the software linked above the normal codecs are often around 20 baud. They're intended to be used with high-frequency radio transmissions, more so for keyboard to keyboard typing interfaces rather than arbitrary digital data but they can easily be used for any arbitrary data.

I've used it on a few occasions to send out a text email or some other kind of small text document. Sending out something like a several megabyte image would be very slow.