According to heroku
Starting today, Heroku accounts have an account-based pool of free dyno hours for use on free apps. This replaces the 18 hours a day limit on free apps, allowing a free app to run 24/7 if needed. New accounts receive 550 free dyno hours and you can verify your identity with a credit card for an additional 450 hours
→ How are dyno hours calculated
If my app is always up, how many days will it stay up for (If I have 550 dyno hours) or does it depend on traffic and usage?
An app consumes hours based on their uptime, no matter the usage or traffic your app receives.
If you do the math, 550 hours is equal to 22.92 days.
If you were to add your payment method, it would give you a thousand hours, enough for a single app to run 24/7.