If my site gets around 100 impressions a day, and my eCPM is, to make up a number, $10, then after 10 days, which means I would have 1000 impressions, do I get $10 dollars?
If you get 100 impressions and 10 clicks each worth $1, your eCPM would be $100, but that doesn't mean that the next 900 impressions will get you 90 clicks each worth $1 each... you could end up with half of that, or more than that. Don't expect after 1000 impressions to get what you originally thought
But even if I don't get 1000 impressions a day, I will still get the CPM amount if I get 1000 impressions over a week, right?
at 100 impressions your ecpm is just a forecast based on the earnings for the first impressions. That being said, Sundays earnings are probably going to be much higher than tuesdays earnings. So if they are calculating it from Sundays 100 impressions you will likely not reach the ecpm. Calculated from a low days earnings would mean that the actually earnings for 1,000 impressions may be much greater.