As much or as little as you like, as Graphene uses a sandbox for Google Play Services. It’s up to the user, some have more or less degoogled completely (like myself), and others use the same apps they did on stock Android, but with harsher permissions.

I’ve not found any real difficulty getting away from the apps I used to use, as there are FOSS alternatives for almost all of them that imo, work much better and require far less personal data (two I use on a daily basis are a NewPipe fork with Sponsorblock functionality, and InnerTune, which has completely replaced Spotify for me.) I do still use Google Maps, albeit with all permissions other than network restricted (as I don’t use real time navigation, and have yet to find an alternative that matches it for business detail accuracy, street view and ease of use.) I also still use Google Camera, albeit without any network or other nonessential permissions.

So basically, Graphene can be used the same way as stock Android. It just gives you options and control over your device and digital privacy. GrapheneOS is first and foremost about device security, and is the best custom OS out there in its field.