[Hydraa]

The dome is 'raised' or 'lowered' based on if there is any unit of a race there that breathes a different atomsphere.
If you have 7999 units on a huge gas giant and add 1 unit of a race that does not breathe the atomsphere. The dome goes up and max facilities and populations limits are reduced.

It is only base on having a different atomsphere race on the planet. No build time or delay in raise or lowering. (Unless you are in s simultaneous game and can not tell your transport to drop 1 type of race and pick up the other race during your movement.)