I had a little look up of this question and the most common answer seemed to be between 18 months and two years.
With small breeds they tend to reach full height between 6-9 months, but do continue to 'fill-out' after that
Ha ha I think he is slowing down, he'll be 6 months in a few weeks. For a 3/4 mini he already seems quite big, must be the JRT genes
i have a mini dachshund cross... she stopped growing at about 9 months... but then filled out. Your guy could have been from standard sized dachshunds... they are quite big in comparison.