Depending on farm size and topology, it will require you ~2 or more hours to change password and test the SharePoint farm and services after password rotation. Once the passwords has been changed in AD, you'll need to update passwords in farm ASAP to minimize the downtime.
Mentioning few places in SharePoint where you must update service account with a new password after password rotation.
Browse your web applications one by one to ensure that all are up and running. Run User Profile Service and Start Incremental Crawl to make sure it works fine. Depending on your farm needs, test the other rest of applications and services.