Your response limit resets each billing period. If you are on a monthly plan, your responses will reset each month. On a yearly plan, you get all of your responses up front at the beginning of your year.
Solo - 20/month or 240/year
Plus - 50/month or 600/year
Team - 150/month or 1,800/year
Premium - 500/month or 6.000/year
You can view how may responses you have left for the billing period by going to the Responses tab and looking for the limit in the top right.