I just figured out how to do this if you want the add-in to contain more than one character using flexbox.
<md-input-container flex> <label>Length</label> <input type="number" ng-model="length" flex="80"> <span flex>seconds</span> </md-input-container>
Important parts of flex="80" for the input element and flex on the span . That says input takes up 80% of the space, and for span to fill in the remainder (at least I think so, I'm still studying flexbox).
The end result is as follows:

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