The short answer is that your tags should start filling impressions immediately after implementation.
Whether or not an ad unit fires an impression depends on multiple factors, however. The domain in question has to be approved into the sovrn exchange, the tags need to be properly aligned to our demand and buying partners, and the unit has to be properly implemented on the page. Where sovrn is placed in the ad stack, the kind and amount of traffic visiting the site, price floors, and ad unit placement on the page also impact the overall fill rate of a tag.
If you could reach out to us directly at firstname.lastname@example.org with your Meridian username we would love to take a closer look at your account.