It seems we don't understand your question. Do you mean How long do they last in your codex? [Forever] or How long can you use the familar? [Depends, but you can simply CC to regain vitality]
They stay in your codex forever. They stay summoned as long as there is vitality remaining, and vitality is consumed when your familiar attacks an enemy.
As long as they don't attack mobs, their duration can be forever. For example: if you go to Mr. Lee's Island and train at the spiked balls and you bring a Jr. Boogie with you, if you position yourself just right, you'll be able to spam your skills while your familiar refills your MP without it losing life.
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It seems we don't understand your question.
Do you mean How long do they last in your codex? [Forever]
or How long can you use the familar? [Depends, but you can simply CC to regain vitality]
They stay in your codex forever. They stay summoned as long as there is vitality remaining, and vitality is consumed when your familiar attacks an enemy.
As long as they don't attack mobs, their duration can be forever.
For example: if you go to Mr. Lee's Island and train at the spiked balls and you bring a Jr. Boogie with you, if you position yourself just right, you'll be able to spam your skills while your familiar refills your MP without it losing life.
I think it varies depending on how much they use their attacks/skills