hi,
if you'll start in the morning at San Jorge/Rivas, you have to take a bus to Managua (not to Granada!) - look for a "Expreso"-bus (the best is the "expreso" from SJdS to Managua (!), Rivas-Managua 1,5hours), if you are not sure, ask the taxi-driver who takes you from San Jorge to Rivas or the people, they will help you very friendly. At Managua you have to change the bus-station - take a taxi to change from
Mercado Roberto Huembes to
Israel Lewites. There you have to take a bus to Leon (1hour), also a "Expreso" or "Minibus" (much faster).

If you'll start at 10a.m. in San Jorge you should arrive probably at Leon about 14 or 15p.m., because you'll get some breakes when you'll waiting for the next bus, the taxi-ride and so on. But be "tranquilo", take some beer or coke, some Nica-food, look at the people's business and take some fotos from this "strange" world...it would be nice and funny!

Have a nice trip, arambigua