long as you have ....more than 12 hrs dark your ok. you just don't wanna have more light period than dark for flowering. but its ok to run at 11 light / 13 dark or 10 light and 14 dark
...but then you will need to check your timer so your lights don't go on or off at the wrong times...thats why 12/12 is best, its at the same time, everyday, for on and off period.