Limit order and opportunity cost

Is there a general rule regarding whether one should enter a limit order vs. a market order, and how far the limit order should be placed away from the market price? A limit buy order saves the difference between the ask and the limit price, but the limit order may never execute. So, I'm wondering if there is a general rule that says, e.g., by placing a limit buy order at the current bid, the order will execute 95% of the time, saving the spread, and more than making up for the opportunity cost associated with the 5% of trades that do not execute. Any thoughts on how to think about this would be appreciated.
 
If you trade frequently, using limit orders should save substantial slippage in the long run for you. If you have a longer-term time horizon, you should use market orders because the slippage will be insignificant. More important, if you use a limit order on a trade that doesn't get filled and that trade would have turned out to be a big winner, it can diminish your profitability and mindset.
 
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