How long will it take to transfer my domain to Porkbun?

Max. time for a domain to transfer from one registrar to another:
five days.

ICANN policy allows for transfers that take less than five days, but it’s entirely up to the old (aka “losing”) registrar as to whether the transfer will occur before five days have elapsed. Some registrars always wait the full five days in an attempt to cut down on domain hijack attempts, for instance when the domain owner’s email account is breached. If you're looking to get a domain transferred to Porkbun as quickly as possible, your best bet is to contact your old registrar's support department and request an expedited transfer. It can't hurt to ask!

The five day maximum is set by ICANN’s rules:

“Failure by the Registrar of Record to respond within five (5) calendar days to a notification from the Registry regarding a transfer request will result in a default “approval” of the transfer.”

source:  https://archive.icann.org/en/transfers/policy-12jul04.htm

When you purchase a transfer, Porkbun sends a confirmation email to the domain’s administrative contact. The five day countdown begins when the transfer is confirmed by clicking the link provided in that email, not at the moment of purchase.