i used to use 24 hrs throut my veg but i found that i do it for the first week when clones jsut get transplanted and then i use 18 on 6 off.What this those inmy experience is not make the plant stretch that much but the root system is fucking nutty, and roots are the most important part of growing pot. I switch them back to 24 hrs just before i enduce flowering but i also give them 48 hrs of darkness. This can stress the plant if not done properly and ifnot given proper food preparing it for such a rough time but the result is a 3 inch stretch and when the lights come back on 12 12 they just kepp reaching for the skye.