It is totally developer dependent. Some apps won't back up at all, some back up only select days, and others do full backups. It's pretty hit and miss.
You can go into Google drive and see how much data each app is backing up but it doesn't give any indication of WHAT data is backed up.
Make sure you're signed in to Google Play Games and you shouldn't have to worry about losing game data
Most apps I really care about tend to have their own cloud backup so it isn't usually a big deal.