The iOS 12 Public Beta is going to release later this month. Apple confirmed this in the iOS 12 announcement post in Newsroom on 4th June.
Once iOS 12 Public Beta is released, average users will also be able to install iOS 12 on their compatible devices without needing a developer account.
Below is quoted text from the Newsroom where Apple mentioned the availability of iOS 12 Public Beta.
Screen Time and the new Notifications and Do Not Disturb features will be available with iOS 12 this fall. The developer preview of iOS 12 is available to Apple Developer Program members at developer.apple.com starting today, and a public beta program will be available to iOS users later this month at beta.apple.com. iOS 12 will be available this fall as a free software update for iPhone 5s and later, all iPad Air and iPad Pro models, iPad 6th generation, iPad 5th generation, iPad mini 2 and later and iPod touch 6th generation. For more information, visit apple.com/ios/ios-12-preview.
The Public Beta builds are more refined and stable than the developer beta releases. If you’ve been holding yourself from the iOS 12 developer beta because of stability issues that might occur, the public beta release just might draw you into the iOS 12 beta club.