Re:Zero is about 40 hours and 21 minutes for the main anime up through the estimate in the source I found, which is roughly 40.35 hours. If you include the short “Break Time” episodes too, it comes to about 44 hours and 10 minutes.
That estimate is based on a breakdown of the seasons, movies/specials, and then an added total for the side episodes. The same source notes the number could go up if later episodes are longer than standard length.
What that means
- Main series only: about 40h 21m.
- With Break Time shorts included: about 44h 10m.
If you want, I can also break it down season by season in a simple table.