It should spell out the allegory clearly, e.g., Galatians 4 where Paul used the images of the bondswoman/Mt Sinai and the freewoman as allegories of the two covenants; one depicting earthly Jerusalem and other the heavenly Jerusalem. Earthly Jerusalem is in bondage, while “the Jerusalem above is free, which is the mother of us all.”
Revelation 2 and 3 have no characteristics of a biblical allegory. The notion that they depict dispensations/ages within a dispensation is pure invention.
It looks like this is the only allegory in the whole Bible. The word appears but once in the KJV.
If you suspected that the number 7 had some specific meaning, how would you find it? It would require that the use of the number is consistent throughout scripture. It would also help if its use in nature is consistent with scripture. Has God done this? There may not be a lot of allegories in scriptures, but it is replete with metaphors, figures, types and shadows. The prophetic character of history in scripture also helps interpret.
Does the allegory in Galatians 4 apply to Abraham's descendants or does it end after one generation?