Trying to be "conservative" is a mistake. You should try to be accurate. How many great and very reasonably priced companies have value investors missed because they were too "conservative".
I agree with Gregmal, you should be looking out at least 3-5 years. Trailing earnings are already priced into the stock. Forward earnings are usually too (though Meta is an example of "conservative" investors missing forward earnings by a mile).
If you look at only forward earnings, every great company will look expensive. If you use trailing earnings, every value trap will look cheap.
Nvidia is a great cautionary tale. In January 2022, TTM (2022) was $4.44. actual fwd (2023) was $3.34. You could have been conservative and estimated 2024 earnings at $3 or $4 or $5. But actual 2024 earnings were $12.96. Sure, you might have missed the drawdown from $300 to $100. But you would also miss the run from $100 to $900. Conservatism comes at a price.
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This also depends on your strategy. If you are buying a cyclical, you should be looking at the past 10-20 years. And then maybe overlay some thoughts on forward earnings. For a real growth stock, you should be looking out 5-10 years.