For private installations this is done by setting the Auth.Cookie.Expire in the UiPath.Orchestrator.dll.conf in 2020 or newer. Prior to this it would have been found in the Web.conf.
It’s been while, I think in 2019 and earlier there might have been a session timeout configuraton from the Tenant > Settings > Security view, but that is while ago if it was configured like that.
Another option would be to look at the IIS Site settings.
If you are talking about Cloud (Community / Enterprise), I’m not aware of this session timeout being exposed. It might be something you need to connect with Support on.