There are a couple of ways to do this. If you have a rate of demand for a product, you can see how many days of stock you have comparing stock on hand to the rate of demand. In the modelling page, select 'Show Toolbar' and 'Hidden Columns.' Tick 'Stock Days' to add this column. Save this new layout for easy access next time. 
If you are in a forecast, open the Supply chart and disable the option 'Forecast Replenishments' to see how the forecast will burn down your current stock. If you don't have any forecast demand, you will need to set the forecast demand in the Demand chart, or by using the Fill Forecast quick action. Learn more about forecasting here.