A 20% annual depreciation correctly calculated will results in a 20% depreciation over a 12m period when calculated correctly.
The depreciation over any other period will vary depending on the depreciation method used.
In particular over 1 month it would only be (20%/12) if simple interest is used (not declining value).
For declining value / compound interest it is a larger percentage per month to achieve the 20% pa specified depreciation.
From memory the correct montly deprecation is
1 - (1 + 20%)^^(1/12)