Definitely not - unless you want some trouble in the form of algae. Once in a while I hear of hobbyists who leave their lighting on for this long and it always surprises me. I hasten to add, they're normally the one's with problems in their tank too.
Lighting duration should be approximately 8 hours per day with no siesta (these don't work). So lets say 1pm to 9pm is a popular time to have the lights on for. If you are suffering from algae problems my first suggestion is to drop the lighting down to 6 hours per day. Similarly for start up tanks, keep your lighting limited to 6 hours as the plants are yet to root and therefore not much growing is happening.
12 hours of light is just excessive. Not only will it cost you more but your lights will need replacing more often and your electricity costs will be higher. Not only that but you will need to clean your tank more often, particularly the glass.
How long do you leave your lighting on for?
