Just used 20 in a row and pulled 2 mermaids...
Most of the time I'll go 20-30 without pulling anything, or when I do pull it's usually Caterpillar, Tiger Lily, or Mermaid. (no Pan or Angel yet)
EDIT: used 20 more and pulled 2 more tweedles (haven't seen them in a while), another mermaid, and another Tiger Lily. Today is my lucky day for pulls, prizes on the other hand...