Yo, its J here,
Ive got a fan blowing on my plants but am not sure wether or not I should keep the fan on all the time? Im using 18 hours light on, 6 off, should the fans always be on.
It depends on a whole range of things.
If it's a little seedling or a clone, you should keep a breeze blowing on or near the plant for a good two or three weeks so the stem thickens up, ready for vegetative growth.
Once it's about 6 inches tall, it depends on what kind of ventilation system you have, and how many plants your growing. If you have a a good ventilation system with a passive air intake, you won't really need an oscillating fan to circulate the air, because the vent system will be doing it. If you're growing a lot of plants and an ineffective venting system, you might need air being circulated by a fan.
There's too many options and scenarios to really give you an answer - it depends on your situation, how many plants your growing and how good your vent system is - this also changes in the flowering stage as well.
I do wish people would give more information when asking for advice.