2mm or less: slight pitting, no visible distortion, disappears rapidly.
2+
2-4mm indent: somewhat deeper pit, no readably detectable distortion, disappears in 10-25 seconds.
3+
4-6mm: pit is noticeably deep. May last more than a minute. Dependent extremity looks swollen and fuller.
4+
6-8mm: pit is very deep. Lasts for 2-5 minutes. Dependent extremity is grossly distorted.
Source: Guelph General Hospital Congestive Heart Failure Pathway