My question is.. supposing that a site brings 13 sales in one week and it gives me a ROI of 160% (i spend 260$ and i made 410$) is this test period enough for safe conclusions that for a longer period of time my campaign will continue bringing such a ROI? besides 13 sales are enough for statistic conclusions? (isn't it an extremely low number?)
You never know, the market is not a constant, it continously changes, if you have more competition in future you will have to up your bids for example, thus making your campaign less profitable. I think from what youv'ee done so far it can be concluded that you're running a profitable campaign, try to optimize it even more. Investing 260$ and making 410$ doesn't seem like 160% ROI to me
Since the market in the internet is always change, its good to promote your products in a website with steady average of visitors each day.
I would be pretty sure it will last for a month or two (maybe more) but you'll never know. Like Ripped said what you have today may be gone tomorrow... Constantly checking and tweaking is what you should do. If it starts losing you money, drop your campaign. Good luck!
I would want about 8 weeks worth to say it was a good, especially with a ROI of 57%. Over the long term, you will have good days and bad days, good weeks and bad weeks. If your one week test was a good week, then a bad week might cost you money. Good job making that much though, most people never make it that far!