When is compression only CPR recommended with no ventilations?

When the rescuer is unwilling or unable to do the ventilations.

"The Canadian Red Cross recognizes that compression-only CPR is an acceptable alternative for those who are unwilling, unable, untrained, or are no longer able to perform full CPR. In some cases, compression-only CPR is the preferred method for members of the public who witness an adult suddenly collapse. The issue has recently emerged based on research published in the journal Circulation."