Global DNS Servers across various countries like
Each server will have a total limit of 20,000 requests with 5 years of validity
Your server will expire once they reach 20,000 requests.
1 person makes 1 query = 1 Request
5 persons makes 2 queries each = 10 Request
So, if you have 5 daily active users with 3 queries each then it will take 15 request each day so 20,000 request will last you about 1333 Days so about 3.65 years.