Maximum number of allowed?

[Anteloper]

I'm running a project that displays my to-do lists as pulled from wunderlist, so I'd like for it to be as up to date as possible. At the moment, I'm hitting the API once every 15 seconds and after a while I get kicked off with the message
"Max retries exceeded with url: /api/v1/lists"

Rather than dialing back the amount I hit the API until it works, I was wondering if you could tell me what the requirements are for exceeding requests. Is it requests per day, per hour?

Thank you.