you want both for veg, preferably more than 12 1/2 hrs also. flowering is typically induced when the suns FR spectrum peaks and drops consistently under between 12 and 13 hrs of light- Peak is morning, Drop is evening. If your peaks are Blue, Blue/White, Yellow/Blue, etc. the plant is less likely to flower. the plants have pigments that detect the amount and kind of light being received, which signals the cycles of its life. Hardened indoor strains always react more swiftly to the switch than landrace or outdoor strains that are accustomed to a cyclical schedule of lighting based on its geological location.
When the switch happens it triggers a reaction of the plant to release stored energy in the form of auxins that formulate the hormones which drive flowering growth, causing a chain reaction in the plant that is seen over the first 2 or 3 weeks of flowering, once consistency has been established the plant will put all of its remaining energy into flowering (to produce offspring) with the expectation of its natural (or conditioned) cycle coming to a close.
Many experiments have been done with very drastically different results when it comes to the plants reactions to going back and forth from in and out of the regular timing schedules, as well as bizarre timing schedules such as 24/12 and 36/12 (or something weird and alternating like 12On-8off-8on-12off-12on-8off-8on-12off), they will reveg and reflower as long as the light schedule stays consistently less than around 13 hrs in a 24 hr cycle, and then consistently more- until the plant has exhausted all of its energy in its confusion and finishes/dies. Very interesting stuff actually, and how well it can be controlled too.... by using LED light technologies it is theoretically possible to send the signal of both flowering and vegging at the same time- never seen it done though or heard of anyone wanting to try lol.