Ive used up to 14 hrs on/off if I had to change my light period due to company or appointments or any other reason someone should be at my house. Noticed no ill effects either way, but cant say I saw any real gain either. ive also never done this for more than a few days (I think the longest I had to do it was 4 days b/c I had a house inspection in the smack dab middle of my lights on cycle). So I dunno. Itd probably be easier if I could find a timer that isnt on a set 24 hr schedual, haha. Its kind of a pain in the ass to remeber to set your timer ahead two hours at 6 am before yor first cup of coffee.