Grow light - duration?

That's as long as I have mine on.. I only increase the amount of light if I want to force anything on!

Today's stupid question ;
do plants need a period of darkness ? 

14 -16 hours a day would be good.  I have mine on a timer.

Can I use a £1 string of LEDs and rig up my own system or are grow lights somehow special?

I have been using grow lights for some years (hi everyone yes I am still here ) I would first ask what is the power rating of the lamps, mine which is used mainly for early sowing of chillies but the tommys go in too is rated at 300w and it is on for 12 hours.
Too much is is a bad thing, I studied Photoperiodism (fave word) and yes as has been stated dark period is very important in the growth of the plant, as chillies mainly grow naturally around the equator this it what I based the light level on.
I would advised you work out what you are growing, how much light do they average in a day in optimum conditions and work from there.
Good luck.

Too much is is a bad thing, I studied Photoperiodism (fave word) and yes as has been stated dark period is very important in the growth of the plant, as chillies mainly grow naturally around the equator this it what I based the light level on.

Very interesting.
Where I live we only get about 3 hours of darkness in mid summer.  Whenever someone mentions the short growing season some other person always replies that things will catch up with all that lovely light in mid-summer. But they don't catch up & then someone else always says it's because the light is at a lower angle/not intense enough. Its one of those "Groundhog Day" conversations that's always the same & I've always thought it doesn't quite ring true.