For the material for which you're checking, note down the stock value. Now move forwards in time & start reducing the stock with the demands you notice. Now on the day on which the stock becomes zero / goes below zero, note down the date. Now check the difference between today's date & the date which you just computed, it would be 54 (Stock day's supply value).
Revert if you still face an issue.
Stock Day's supply calculation is done based on setting in Customizing for MRP or for Consumption-Based Planning in the IMG activity Define receipt elements for days' supply calculation.
Check for your plant related settings in above Customizing settings. Here You can include required receipt elements which can be included to calculate stock day's supply.
Even you can get more details through F1 help.
Revert in case any doubt.