Do the verb tenses look wonky to you? They sure look that way to me…
In first-person narration, the tense may go from past to present and back without warning if the narrating character drops out of the story to address an audience (listener or reader). It’s sometimes appropriate and sometimes not, and it’s not really possible to tell for certain from such a small sample if the author meant to do it that way or if the present-tense sentence is a mistake.
Here’s the all-past-tense version: How long had I been here? There was no way of knowing. I used to scratch the days on the walls, but it seemed pointless after a while.
Whatever is done with the verb tenses, there does need to be a comma after walls, because the last sentence is compound.