It’s fascinating how similar my experience was to yours. This summer I did a seven days retreat (five full days). It was my first one
I was also counting days, but, by day five, I adjusted and then caught myself thinking that I could have kept going for weeks. Then, at the end of the fifth day, we broke silence for about two hours. As I started hearing conversations here and there, my mind quickly snapped back to counting hours before the end.
More recently I went on a one day retreat. The retreat went from 9:30AM to 4PM and it was a very pleasant experience.
My intention now is to practice daily, go to one day retreats every once in a while and do a longer retreat once a year.