Every 7 days you can summon new elemental guardians without it make previous ones get disappear. You can make a huge army of guardians. It seems odd. Doesn't it lead to any trouble?

Oh, it limited to 5 guardians. Is there any way to expire them? Or the only way is they die?

If you are using the Co8 mod, you might be able to go to Jaroo in Homlett and ask him about spell permanency.
Doing so will likely either dispel the elementals or make them permanent.

I do know if you dominate any NPCs with the spell of that name then talk to Jaroo, the domination becomes permanent.