This really applies to your product or service so my suggestion is going to be general. Measure ROI on a monthly basis. You want to review it often enough to react to new trends, but you also don't want to over tweak or react to speed bumps.
Yup, I thought a month sounded like a good period of time to me. I was just interested in seeing what other people used.
depends on how much it costs you per day and how much you can make off a conversion if you make $10 per conversion and you sell 10 in 1 day after you spend $10 on advertising, this is a strong sign things are going well if you spend $100 and sell 0 after 1 day it looks like things aren't gonna work if you spend $10 over 3 months, and make 1 conversion, then who knows whether its working well or not, you'll have to try testing more
This really applies to your product or service so my suggestion is going to be general. Measure ROI on a monthly basis. You want to review it often enough to react to new trends
the longer you accumulate data the more accurate the "average" becomes so if you continually keep analyzing over a longer period you will have the best results