Hmm..
When figuring out lighting time for your fish, try and google where they are naturally from, then look up the sunrise and sunset times for that area to get an idea of how much light they're naturally exposed to, as a base to go off of, since lighting is a part of your fishes health.
Guppies need 6-8 hrs or rest period a day, so doing 12 hrs of light a day would be fine for them. If you start having algae growth, cut back to 8 on 16 off like
@The Lumpfish Guy said, or even 10 on 14 off. Don't forget lighting is just 1 piece of algae growth too.
Do you have plants? If not, it wouldnt be a bad idea to figure out what your wattage per gallon or lumens per litre is, that way if you want to have plants later on you'll know what kind of plants you can get based on what level of lighting you have (also consider water conditions when choosing plants).
Another tip I guess - over time evaporation can cause buildup on lights, so can algae, so every now and then during water change dip a rag into your old tank water and scrub the light clean, helps that crystal clear water shine.
If they're LEDs, over time they will lose their brightness but still work, when you eventually hit that point (way down the road), be sure to replace them to maintain healthy lighting (especially if you have plants at that point).
Hope this helps, I'm still new-ish to the hobby I should warn
=P