As someone that bikes with a power meter, looking at my power curve for some rides: I've held 300W avg for 20 minutes, 200W avg for 3 hours and 160W avg for 9 hours. These are from races, a steady state effort could probably be a bit higher.

I think that may be some usable numbers for what an avid amateur cyclist can achieve (ftp ~4Ws/kg). And then one will have to take into account that the body efficiency of cycling is about 25%, so one will have to eat 4x that amount in calories...