I likely added additional confusion or did not actually answer your question. Lets call it simply queries. 20,000 queries over the course of the period. For the Silver plan you are able to receive up to 150 responses per query, however, each 15 responses counts as a query. Therefore if you allow your query to return 150 responses, you would actually use 10 queries.
For your example, lets say there is 10 responses for each query every 10 seconds, you would use ~260,000 queries, over 10x the allotted amount for the Silver plan. 2592000 seconds in a month/10 (1 query every 10 seconds) = 259,200.
If there were 100 responses for each query every 10 seconds, you would use about 1.8 million queries. (2592000 seconds in a month/10 (1 query every 10 seconds))*7 (100 responses/15 (responses per each query) rounded up = 1,820,000.
It would depend on how many responses you are expecting for each query.
For Silver to work and for less than 15 responses per query, you would need to query just over every 2 minutes.