Siamese fighting fish do not need much in the way of aquarium lighting. If you can see your fish, you've lit it sufficiently. However, lighting does have some specific effects on Siamese fighting fish that every aquarium hobbyist should be aware of.
In terms of viewing the fish, you need very little light. Siamese fighting fish do just fine on ambient light without additional aquarium lights since they originate in dim, vegetation-choked streams. However, you may still want extra lighting so you can see if anything's wrong with your betta. For example, bettas are prone to velvet, a common aquarium disease, but it can be hard to spot the symptoms of velvet under dim lighting.
How Much Light
In general, fish-only aquariums need very little light. A well-lit fish-only aquarium -- the kind a Siamese fighting fish can thrive in -- needs less than 1.5 watts of light per gallon of water. However, if you keep live aquarium plants with your betta, you'll need more lighting -- somewhere between 1.5 and 5.0 watts per gallon. Keep in mind that bettas do not need to share their tanks with live plants to thrive, so adding plants to their aquariums is not necessary and can make its upkeep more complicated.
It's also important to turn your light off at night. Bettas have no eyelids, so an aquarium light glaring 24 hours a day can make it hard for them to sleep. It's usually easiest to match your fish's light/dark cycle to your own. Use an electronic timer to make this an easier task. Since bettas come Thailand -- formerly Siam -- they have evolved in a tropical photoperiod. Having the lights on for 10 to 12 hours per day simulates their natural habitat well.
Problems With Extra Light
Too much light can cause problems. For example, excessive aquarium light can promote the growth of algae. If you leave your aquarium lights on constantly, algae can thrive in a betta's tank. Additionally, this same problem often occurs in aquariums positioned in direct sunlight. If you find yourself having trouble with algae in your betta's aquarium, decrease the amount of light the tank gets, either by blocking out direct sunlight or limiting the hours per day you run the aquarium lights.