To give you a bit of perspective: in other industries, it's quite common that Assessment Center sessions last around 6-8 hours (however, there are breaks and lunch included) and it's not uncommon to see even 2-day assessments.

Is it fair? I'd say yes, if following 2 conditions are met: 1. The method (AC/task/exercise) is well designed to actually test the candidate's skills and his/her performance is properly observed, measured, analyzed and assessed. It's fair for the candidate, because the hiring decision is adequate. 2. After all this effort the candidate receives both a "thank you" and short feedback about his/her performance.

Unfortunately these conditions aren't met quite often and that's not fair.