Some of this depends on crawl rate and the number of pages you are aiming to de-index. We ran an exercise in which we aimed to de-index 40k pages from our site. It took two months before we saw at least 50% of the pages de-indexed, while it was 6 months before we even saw 75% of the pages de-indexed. However, these were pages that were part of a subdomain with very little authority and low crawl rates. We performed the same exercise with a smaller set of pages, but much higher crawl rates and higher domain authority, and saw an almost instantaneous (24-48 hour) impact in which they were removed. As the mobile site sounds fairly new, you may just have to be patient on it as it will likely take longer to de-index.
