It usually doesn't take me long to get my rare critter to appear. Ive piped 12 Red Slimes (Redria = Lavis) and seen 3 naturally... The longest one was 2 1/2 hours. The rest are generally 1 hour or a bit more. But one episode of bad luck did eat up 8 hours of my time. I tried piping Skyly VH Hildeblue's (FoS 4 Hildes at a time) for God/Arms. I piped for 2 hours, for 4 days straight, not to see a single Hildeblue... Thats the only bad one i have. Ive since found a MUCH MUCH better way to find God/Arms
Thing i have with piping is, once you've been doing it for 2+ hours, you KNOW its gotta come very soon. So you keep going and going... 9 hours without a rare enemy, i would be tearing my hair out