So today I decided to try switching to a different Grand Company. It did say I could transfer back after a certain period of time. After switching, I asked the npc and he says I have to wait 30 days before I can transfer back. Is the cooldown for switching Grand Companies really 30 days?

By the way, when you switch you don't lose your seals or rank they just get locked. You also can't use your old grand companies gear. You just start from scratch in the new grand company and have to rank up from the bottom.

For those who want to know how to switch to a different grand company, you have to finish all the story quests, be level 50, and clear some dungeons. Then you can get promoted to 2nd Lieutenant (which is the highest rank available right now) where you get the option to switch.