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 Friday evenings to allow for the propagation to take place over the weekend.