Assuming you are checking your account with a credit card, you have 1000 free dinos per month for the game.
Double hour is just one hour of dinosaur work.
If you have only 1 application running on just 1 dynamo, this dino can be available 24/7 forever, since even a 31-day month consists of 31 x 24 = 744 hours, which is less than 1000 free dinos at your disposal.
However, if your single speaker in your single application is a “web dino” (ie a web server), then note that free web dinos sleep after 30 minutes of inactivity (in this case, the next request to the web dino "wakes up").
The free web sandwich is a double-edged sword. On the one hand, when web dinosaur is sleeping, it does not consume any of your free dinos (so you can get more for your 1000 free dinos, depending on your needs). If you can tolerate a slow response to your web clients in response to any requests that arise to wake a sleeping dyno - this may be OK. If not, you will need to make sure your web speaker is busy (that is, make sure it does not go 30 minutes without receiving any requests).
See the Dyno Free Clock for more details.
Please note that dyno is just a virtual machine. This is your "server". A single dinar can certainly support many users, depending on the complexity and performance requirements of your application. You can "scale" your application both "vertically" (which means increasing computational capabilities on dino) and "horizontally" (which means launching several instances of dyno that load the balance of your application’s traffic). See What is “Dyno” Heroku? . Your application must handle absolutely GIGANTIC traffic to justify dyno scaling up to 1000 instances. If you really need to scale your dinosaurs (it is unlikely if you are just starting to create your application), you will need to use paid dinodes.