Typically speaking, propagation can be completed within a couple of hours. You should receive a disclaimer from your registrar when making the DNS change, and their disclaimer will be a better representation of what you can expect. Note that in some cases it can take up to 72 hours to fully propagate. You may experience difficulties accessing your domain-associated email during the transition.
We recommend that changes to nameservers (DNS) be made on Friday evenings to allow propagation over the weekend.