The time it takes to transfer a domain varies based on the type of domain name it is. With .uk domains, your IPSTAG change request needs to be processed by your existing registrar.
Once it has been processed, then your domain is transferred into your account immediately. With gTLD domains, an authorisation email is sent to the administrative contact on the domain name. In this email, the new registrar asks the contact to provide the authorisation code, which should have been received from the old registrar.
The old registrar verifies that the authorisation code is correct and the old registrar asks the owner to confirm that they want to transfer. Once the owner confirms, the transfer is completed. Due to this process, the transfer of a gTLD domain usually completes in around five to seven days.
With other domain names, the process for transfer can vary, but usually take approximately seven days to be processed. If you are switching nameservers at the same time, DNS propagation times will apply. This is usually 3-6 hours, but can potentially be up to 48 hours across the entire Internet.