Sponsored Product ads on Instacart have an attribution window of 14 days. This means that a purchase will be attributed to your ad if a customer purchases that product within 14 days of clicking.
Instacart gives users the choice between a linear multi-touch or last-touch attribution model. The default attribution model will be linear multi-touch.
Linear multi-touch attribution distributes the sale across all ads that a customer interacts with leading up to the purchase. The last touch model will assign credit to the last ad interacted with before the purchase.
Trends on Instacart show that shoppers typically fill up their cart throughout the week, and check out at the end of the week. This means you might see your ROAS increase at the end of the week.