I think we have several more years of Adriana, 3 more on her angel contract. Then with her signing her new beauty contract with 4 more years left pretty much means she'll renew her angel contract again once both expire. I'm guessing we have about 5-7 more years to go. But I believe the work will get less and less, but right now she's in her prime, she's one of the most famous faces they have. Which is why they are preparing for her comeback and want her to take part in everything they do. Heck they still have her doing appearances while she's pregnant! So she's a very important part to the VS team. I'm sure in the next 7 years we'll still see plenty of her before she goes ahead and retires...I wouldn't count her out quite yet.
Plus remember VS keeps Heidi around because she's their biggest fame name, she draws in a crowd. Adriana is only second to Heidi in that, which is probably why they are being used this year in the fashion show. Because as much as I like Ale, Marisa and etc, while they have their own fame, they don't have the name power like Heidi does or Adriana (marisa's pretty close) So they don't keep them around for nothing, plus the public (celebrities included) love Heidi and Adriana and they pull in a lot of people for the fashion show...so it's good business.