It depends one several factors--type of engine, how far you ride, the climate, how aggressively you ride, type/quality of gas/oil you use, and maintenance history (and other things, I'm not thinking of).
For example, I live in a 4-season climate, and put my bike into storage for about half the year. So, in March (optimistic) I uncover my bike, put air in the tires, change the oil, lube the chain, gears and other moving parts. Then I take it easy for the first 100 miles. Then I ride it the rest of the season with only minor maintenance like lubing chain, maintaining correct PSI in tires, and topping off oil if needed. If I am I not riding it regularly, I will connect a charger to the battery to maintain charge.
At the end of the season, I change the oil, lube the chain, gears, and other moving parts, check PSI in tires, connect a charger to the battery, top off the fuel tank and add a fuel stabilizer. Then I cover it up and cry for the next 5 months until I can ride it again.
I don't ride aggressively, and almost always on pavement (the occasional gravel parking lot), so I'm not really hard on the bike. If you were to do a lot of riding in a dusty area or if you just ride A LOT of miles in a season, for example, you would probably need to change your oil more frequently.
At some point, you may need to check the spark plug and readjust the gap (or replace it altogether). Some bikes and riding styles will need that sooner than others.
So with all that in mind, changing the oil every 6 months doesn't seem unreasonable to me. If you are not pushing the bike really hard, and are keeping up with all the other servicing needs (lubing the chain and gears, etc.), then you might be fine with changing the oil just once a year. Keep in mind that oil is affected not just by miles ridden, but also by time it has spent sitting in the engine.
Personally, if I were riding year-round I would change every 6 months. If you ride half the year like me, once a year (at the end of the riding season) would probably be enough. I just like to be proactive.
You might try to get your hands on another service manual. Is there a dealer near you that might have one? If they don't have any for sale, maybe they would at least let you look at the book.
Try searching online for the manual, too. This site has many manuals - http://www.carlsalter.com/motorcycle-manuals.asp . You might also try searching for "hero honda motorcycle forum" and see if there are other owners you can talk to.
Second to manual, listen to your bike. Is shifting harder? Are you losing power in lower gears? Is the clutch sticking? Is the engine running hotter? Louder? Does the steering feel "loose"? All of those things can alert you that your bike needs attention.
Ride safe! :)