Depending on your requirements, OpenAthens can be configured to work in parallel with your existing on-site authentication system. By doing so, your users will use OpenAthens only when they access library resources remotely.
Alternatively, you could choose to implement OpenAthens as the only access management method. This means users will be prompted to login on and off-site. The benefit of that type of implementation is that it will give you an accurate view of resource usage, all recorded and visualized by OpenAthens.
In any case, users will trigger an OpenAthens session the first time they are asked to authenticate. The session lasts for 8 hours before there is a need to authenticate again.