Shard Collection Settings

I have a question about sharding the collection.
Currently, tens of thousands of data are stored in the collection.
I want to change this collection to shard collection.
I wonder if the previously stored data is automatically distributed and stored if I change it to Shard collection.
And if you change the collection where approximately 100GB of data is stored to shard, approximately how long will sharding be completed?

That is impossible to tell.

It all depends on your configuration; how many machine, how many shards, network connections between machines, storage type.