How often should I change the oil in my lawn mower
It (probably) won't suddenly stop and refuse to start again after 5 hours of use, but it's a good idea to follow the recommended cycle: the parts in new engines often haven't been milled perfectly smooth, so as your new engine turns those rough bits get knocked off or cut tracks for themselves, leaving little pieces of metal in the oil.  Unless you get rid of them by changing the oil, they can cause excessive wear and tear on the engine.

Depending on your lawnmower, you'll either be able to drain oil out of the bottom, or tip the mower over and drain it out the top from the same hole you pour the fresh oil into (the worst part of this is lifting and tilting the mower over the catcher pan, but it's a lot easier if you have someone to help).