Email warmup is a process that improves the reputation of a new (or long-inactive) email account in order to increase its email deliverability.
This process involves various activities such as: sending (and gradually increasing) a small number of emails each day, replying to them, marking them as important, etc.
Ideally, you want to warm up your email continuously. You need to warm up your email address at least 4 to 8 weeks before starting your outreach.
It depends on the scale at which you want to send emails: