do you have a minimum earn amount before you quit?

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I've been active on a couple of PTC sites for months now and not made the minimum payout. I am talking about a few dollars, not the first payout at $25 dollar sites. I admit that the activity hasn't been constant and it's not that i think the sites are scams, i just don't think it's worth the time each day to log in. When you you chose to quit, when does your time become worth more than the payout?