Good question, as it varies from species to species. I consider it mature when the growth form is clearly defined. By your piece, if it is A. loripes, then it will continue to grow up rather than out, while if it is A. granulosa, it should start to table. From the angle of the picture, it is unclear in which direction the main growth is folowing. I see branching coming near the base, but I see growth at the top, as well. Both qualities are more A. loripes, but I have seen A. granulosa looking similar until is suddenly grew only from the edges and base. Current and wave action can strongly influence the growth form, even in the aquarium.
In any case, these species require the same conditions.