How long are call logs stored and is this configurable? As far as I can tell, my OnePlus 3 has complete call logs for the past 6 months (last log 4-19-2018, today is 10-20-2018 which seems too much of a coincidence)

Logs are really useful but some calls I make only once a year so being able to keep (more) complete logs would be really useful.

Is this something that's configurable on Android? And, is there a way to sync this with my Google account so it's backed up even?
There is a limit of 500 entries for calls (incoming, outgoing and missed) in Android. How long will the call log last, go back in time or get filled depends on how often you call or are called.

If your device is rooted, you can install Unlimited CallLog for LP/MM/N, an Xposed module that extends the limit to an unlimited number like its name suggests for Android 5 to 7. For non rooted devices, a search via Google Play Store provides a list of apps that should also extend the limit.

Your device should have the ability to backup the call history; search for Backup in the Settings app. Some of the apps (see previous link) also provides for backup options.

Manage &amp; restore your device backups in Google Drive