alt Hacker NewsAsymptotic models are not a statement of how long it will take; they are classes of functions modeling the relationship between some performance outcome (run time most often, but sometimes memory or other things) and data coming in. These mappings from input to performance characteristics are not conditioned on physical limitations but are rather abstract tools for measuring that relationship.
If you have an instance (which is what I assume is implied by "real-life data structure") bounded by a particular size, then you could say that the runtime complexity is constant with respect to that bound. Whether that is a useful statement to make is another discussion entirely. On its own probably not. It can potentially be useful for analyzing the runtime complexity of operations in more complex algorithms that use this data structure up to a certain fixed size, e.g. as a buffer/cache of some sort.