Working on the basis of a "month" being 30 days, because if you assume 28 (aka 4 weeks) it creates a 13th month, although even on that basis the max you reach is 5 months not 5.5:
6.0-6.1: 18 weeks (4.2 months)
6.1-6.2: 19 weeks (4.4 months)
6.2-6.3: 20 weeks (4.6 months)
6.3-6.4: 19 weeks (4.4 months)
6.4-6.5: 20 weeks (4.6 months)
6.5-7.0: 39 weeks (9.1 months)
To replicate these results do: weeks x 7 divided by 30.
So it seems to vary between 4.4 and 4.6 months depending on holidays, while expansion launches I know have increased by 3 months for the last two expansion releases (which was explained for Endwalker due to covid but not for Dawntrail).