Looks like a new plant to me.

Looks like an agave pup to me. Wait a few months and it'll turn into a tiny agave plant: when mama plant needs to be repotted,.you can cut away the pup and plant it separately. I will post photos of my agaves and their pups later.

But, don't disturb the pup for a long time yet, it is small and delicate ;)

How or when do you know when to replant???

As Kat has said, you have to wait until the pup has its own roots and it will take some time. Enjoy your plant for now.