Fixed Asset Depreciation Question

Guys, wondering if any of you had a way to approximate the remaining monthly depreication expense on a group of fixed assets when the purchase date & useful life are unknown. What is known is the ending balance sheet values for Gross PP&E and accumulated depreciation, plus LTM monthly depreciation. I know there isn’t an exact value but something needed to closely resemble the expense so future cash flows are impacted adversely. Thanks.

If the assets are the property of a going concern entity you can reasonably assume that old assets will need to be replaced with new similar assets… so if your company’s projections are flat for the next few years, why not simply use your LTM monthly depreciation (or the average of the 12 datapoints) as your projected depreciation? If you are projecting some revenue/profit growth or decline, you might want to tweak it so that monthly depreciation grows/shrinks accordingly. Rough, but explainable right?