There is no definite or perfect scale; but some scales are usual:

• a linear scale over days
  1d, 2d, 3d, …, 10d
  useful for non-real time recovery scenarios; that is when it takes days to recover service
• a short exponential scale over a range
  4h, 12h, 2d, 7d, 15d
  useful for interactive systems where recovery is as fast as possible
• an exponential scale over a range
  10m, 30m, 1h, 3h, 6h, 1d, 2d, 7d, 10d, 14d
  useful for interactive systems where recovery is as fast as possible
• an exponential scale over a large range
  15s, 1m, 30m, 1h, 6h, 12h, 24h, 2d, 7d, 30d
  useful for interactive systems where recovery is as fast as possible, but it might take days or weeks to be recovered