As a rule of thumb, Lookalikes should be based on your best quality seed audience with at least 1,000 members.

As soon as you have 1,000 visitors, use that as your seed. As soon as you have 1,000 add-to-carts, that should be your seed. And as soon as you have 1,000 buyers, that should be your seed.

You can try experimenting with different seeds/LLAs simultaneously too as the numbers get close.

A final tip, it almost always makes sense to use max a 180-days time window for seeding lookalikes.