This depends. For a new mattress a period of adaptation is entirely normal and varies from person to person. In general, there are those who sense the benefits after only a few days, while others need weeks. In any case, a period of adaptation is entirely normal, above all when switching to an entirely new bed, frame included, or when switching to a new typology of mattress, for example from a traditional spring model to a Memory foam mattress.
Even the substitution of a very old mattress can extend the period of adaptation as our body may have memorised an incorrect position, caused by the physiological deformation of the old mattress..