Does it have to be completely dark when i'm vegging?

If i go 18/6 for veg does it have to be completely dark for the 6 or just when i go 12/12 for flowering?

I assume it doesn't since some people go 24 on but just want to make sure.

Thanks

Just 12/12

if that was me i would make sure they are in total darkness n both veggin and flowering stages

they must be in complete darkness during dark hrs, it will fuck up their cycle if they are getting less light sometimes then other times, and you will end up w hermies