Using uniform units, 0.25 μSv acute dose is what the manufacturers claim for these devices. Annual average background from all sources in US (including these) 6,000 μSv/yr. So you need a scan every 21 minutes for a year to roughly double your annual dose. In some parts of the world, the natural background rate is higher. Like 50,000 μSv/yr. Cancers or other harm have never been shown to occur (or not occur) below acute or annual doses of about 100,000 μSv.

So you need 400,000 of these scans (at the manufactuer-claimed dose of μSv) to reach a dose that is thought to have an above-zero increase cancer risk.