It may be reasonable to estimate from DM date than Test date.
Yes, time from DM to oath invite is normally pretty consistent at between a few days to one month for the offices that are not super slow. But in super slow offices, especially for 2020 and 2019 applicants, it can take 10 months.
From test to oath invite is less consistent indeed. It tends to be 2-5 months for 2021 applicants, less for 2022 applicants, but also depends on how long it has been since the application was submitted. For 2020 and 2019 applications, it can be 1-2 years or more. For me personally, it was 5.5 months (6.5 months from test to oath ceremony), but I suffered an unusually long delay between DM and oath invite.