We try to assign the magazines we cover regularly to an indexer as close to when they appear on the newsstand or in subscribers' mailboxes as possible, but it depends on the magazine and how closely they stick to a consistent publication schedule. And we ask indexers to turn magazine indexes around as quickly as possible so that we can approve the index in a timely manner.
The Mar/Apr issue of Cook's Illustrated is Indexing Now as the record was just created in the Library/Recipes and assigned to the indexer, and it will be Indexed in the very near future. If you've "subscribed" on EYB to Cook's Illustrated, the new issue will have been added to your Bookshelf automatically, or you can add just the Mar/Apr issue if you prefer.