1. Don't overeat at meal time.
2. Don't overeat at meal times.
3. Don't overeat at mealtime.
4. Don't overeat at mealtimes.
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Which one is commonly used? Can we use all the expressions?
I'd use #3 only.
http://www.merriam-webster.com/dictionary/mealtime
All of the expressions you provided can be used, but the most commonly used expression is "Don't overeat at mealtimes."
Here's how you can determine this by conducting a simple online search:
1. Use a search engine(e.g., Google) to search for each expression by typing it within quotation marks.
2. Compare the number of search results for each expression.
3. The expression with the highest number of search results indicates its level of common usage.
By following these steps, you can determine the frequency of usage for each expression and conclude that "Don't overeat at mealtimes" is the most commonly used expression.