180 day remarketing duration is not good enough.
AdWords and ALL other digital marketing platforms allow lists to have NO duration.
This means there is no way to measure or divide up purchases through Bing that are from new customers compared to past customers.
For example, let's say I have two different campaigns, one targeting past customer purchases, one targeting first time purchases. The first campaign has a list of all past customers attached to it, making sure that this ONLY tracks 'existing purchases'. The second campaign excludes this list, making sure this ONLY tracks 'first time purchases'.
After 180 days, past customers will be removed from the list attached to the first campaign. Therefore, they will be picked up unsuspectingly by the second campaign and Bing will class them as 'first time purchases'.
I find it baffling this hasn't been brought to your attention sooner, as it is a major flaw in your platform. As I said, AdWords, Twitter, LinkedIn and ALL other platforms do this already. As a result we continuously keep lowering our company budget for Bing and shifting it to these other platforms.
Please review this as soon as you can as it is messing up all our figures, and (I suspect) many other company's figures too.