Nice, but: you're not using the SE provided Imgur account?
But then you're basically introducing image rot?
For free accounts:
How long do you keep the images?
As long as images are getting at least 1 view every 3 months, they will stick around forever. After that, your image may be removed to create more space for newer images.
(Note that a view is a request by a browser that is not using an image from its cache. Imgur tells browsers to cache a full year. I doubt Google indexing will keep refreshing the view count? Of course, unless edited by the OP within the grace period, the post revision history will still refer to the first image, which is probably hosted on the SE provided account.)