totally up to your grow plan.. If you have 20 photo plants I would veg for 4 weeks or less.. If you have 4 then I'd say push em farther.. But with age comes stretch so you can't have 2-3 months of veg without your plants getting very tall in our smaller growing spaces. The limitation is really your own ideas and wants. plus your grow space.. Keep in mind depending on the genetics they can stretch anywhere form 100%-300%.. So a 1 foot tall plant in veg could turn into a 4 foot tall Sativa monster..