It's really impossible to say unless we know the load put on the genset.
Kind of like asking how many MPG will I get with my V8 truck. Are you towing or solo. What gears? How fast do you drive? How heavy?
Now, for lets say half load on your genset you will get somewhere around 9 hours out of a 20LB tank. (about 10 to 15% less on propane then on gasoline)
Be aware a genset can't produce the same shaft horse Power on propane that it can on gasoline. IOW's if you max your genset out on gasoline you will be short on power with propane.