Usually my rule is... when I use the seam ripper twice in one day... It's time to quit.
On a side note... I had moved my sewing machine into the dining room while my sewing room was under construction. I dropped my seam ripper, it stuck in the linoleum and BROKE THE TIP! WAHHH It was my favorite seam ripper too. Do you know how worthless a seam ripper is without the tip?! VERY worthless. I was sad. Luckily I have back ups!