How big of a pot do you have it in? or what is it in?
The idea of veg is to grow the plant to the perfect size to where when you flip the lights and it stretches it fills the footprint of your light perfectly. No waste. There are people who say they've kept the same mother plant for 10+ years. It is possible to keep a plant in alive indefinitely. Even after you flower it, if it hasn't been pollenated, you can switch it back to 18/6, or whatever you're into, and re-veg it. Marijuana is a annual, meaning that it dies back after pollination. If it never gets pollenated it will grow like a perennial.
But 12 week veg is a long time indoors. I usually do 6 weeks. The main idea there is that the more you yield off of a shorter veg, the cheaper it was to produce it.
got any pics? I bet it's a monster.