I recently joined a online panel for online market research surveys, but last week reviewed one after completion for additional payment for some test products.
In the product consent form and letter for the product it never mentioned upon completion when the balance will be credited into your account, unfortunately the FAQ and email section is not working as it is Christmas and won't be having any answers soon.
if other who has exceptional experience could come forward and give a statement will be good. I did find which it could take 30 days as it is only thing that comes up on Reddit forum, correct me if I am wrong.
I have done it before in my own free time and it isn't worth it. The whole $16.80 an hour is complete BS as that's what you would get if you were doing surveys for an hour. I've never really had an issue with receiving additional payment or just payment in general to an account. It was always instant.