how often does mutation in cells occur?

Question: how often does mutation in cells occur?
- Keywords:
  cell,
  mutation
Asked by keep49hue to Marcello on 18 Nov 2020.
- Marcello Valente answered on 18 Nov 2020:
  
  Dear Keep49hue,
  in simple organism like bacteria we have an average mutation of 1 over 1000.000 sequence in their DNA.
  In complex organisms like us it’s 1 over 1000.000.000 and we also have defense systems to keep mutation at bay.
  Uunfortunalely over time this mechanism start to fail

Question: how often does mutation in cells occur?