I would put no time limit of switching sides. I know that enables unscrupulous players to look at where CV's are, but I don't care as much about that as the advantages of being able to even up sides. If it's a CV near any action, the enemy already knows where it is anyway.
That way, you can always switch over to even up the numbers, but be assured you can switch back after things are more even.
The lags in being able to switch are what cause me to stay on a side with a large numbers advantage. Instead of switching (and possibly getting precluded from switching back), I just start flying earlier-war planes or taking up A-20's instead of fighters or flying He 111's in at 5k.