how old do they fully develope their labyrinth?

[ICEAL]

when can i take the plastic cover off?

[1fish2fish]

It's up to you really. I prefer to leave mine on as long as possible. Humid air is even best for adult bettas so there's really no right or wrong time to remove the plastic if that's what you're using.

[bettafish15]

As long as the plastic cover has air holes, you keep it on until they are at least adults I would think, then you can take it off if it bugs you...

[Curlyfatbottom]

from what i hear as early as 4 week old.
but i don't take my off until they hit a 1in. long