Generally, skilled crew and regularly inspected tested equipment make this a rare occurrence, and it is not unheard of for assistants to go for months without seeing one, even on large-scale shoots.
The cinematographer Oliver Stapleton praised his assisting team on Casanova for not having a single hair or scratch for the whole shoot, even though large portions were shot on dusty exteriors.
The Arri D-20 and D-21 system, however, does have removable lenses and a rotating optical shutter, which means that the CMOS sensor can be exposed in much the same way as a film gate and thus needs to be kept assiduously clean.
In this method, the lens is taken off of the camera and the shutter is moved either with an automatic button (usually labeled "phase" or "dust check") or by manually inching it to an exposed position.
The lens itself acts as a large magnifying glass for the focus puller, who only needs to position the flashlight and their line of sight properly to scan the gate.