How did you pick this value? Do you know why the results would be so sensitive to how long the lookback is?
I experimented with this extensively and found it made no difference. If you are finding it is making a big difference, perhaps:
- Check you're calculating the volatility of daily returns, not the volatility of price.
- Check you've shifted your returns vector backwards a day before you calculate today's volatility, otherwise you introduce lookahead bias, which will be inversely proportional to the lookback time.
If you have a Sharpe ratio of 0.8 for a single instrument over 30+ years, that's too high- something's wrong. 0.3 is more likely.