How long should I run cart abandonment tests?

I've been running my Shopify store for about 18 months now, and cart abandonment is driving me absolutely crazy. We're spending thousands on Meta ads, getting great traffic, but seeing around 70% of potential customers drop off at the cart stage. I've tried generic pop-ups, generic discount codes, but nothing seems to move the needle consistently. Our average order value is stuck around $85, and I know we're leaving serious money on the table. I need a systematic approach to understanding why customers are bouncing and how to effectively recover those lost sales. The frustrating part is that I can see these customers are interested—they've added products to the cart—but something is stopping them from completing the purchase. Is it pricing? Shipping costs? Lack of urgency? I want a methodical way to test cart abandonment strategies without burning through my marketing budget or annoying potential customers with aggressive tactics.