when do i change the light cycle

im growing sum blue cheese under 400w hps when do i change the light cycle and do i need ventilation through out the whole grow?

you can change the light cycle whenever you want to. most people usually wait anywhere from 2-6 weeks i would have to say. the longer you wait the bigger the plant and higher the yield. also ventilation is a good idea. it can help lower you temps if they are too high, it brings fresh air into the room, and the plants stems will grow a bit thicker due to having to resist the moving air.

cheers mate their 5 days old do i need ventilation from day 1?

Only your thermometer can tell you that. Ideal temp is around 26 degrees celcius, or 80 degrees fahrenheit.