I can't remember how old I was, but I'm pretty sure it was when Futurama was still airing in its regular run, though I don't remember the episode. I was over at my cousins' and they were watching it and I just remember thinking it was really stupid, not funny, and I didn't get why Leela had only one eye and it pissed me off (so I must've been fairly young then).
The second episode I watched, a few years later, was Amazon Women in the Mood which I thought was absolutely hysterical and I loved it. The third was The Luck of the Fryrish and that sealed it for me.
Similar to what happened to me. I saw "Parasites Lost" and thought this is pretty funny....then a few days later I saw "Fryrish". That one totally blew me away.