Lighting timing etc

I have been growing outdoors and have now decided to grow inside, however I do not know just how long the light should be used for? I have a small area, 2ft x 3ft with a 250w HPS.
I have been told various timings from 18 to 12 hours per day?
Can someone please let me know how long to keep the light on? and as the 'erb grows to I increase or decrease timings?

 

many thanks.....will let you know the results.
Using "Mazar" seeds.