If you are using their $5/month plan than you get 20 months [1]. Obviously backups and other things will add a little to the cost but realistically easily a year of free server time.

[1] - https://www.digitalocean.com/pricing/

About 20 months of their $5/mo plan: https://www.digitalocean.com/pricing/

If you run the cheapest one at $5/month then almost 2 years.