“Over” and “during” are not synonymous when used for time: “over this period” means from a point at the beginning of the time period to a point at the end of the time period; “during this period” means within the time period.

Example: During this period, prices fluctuated wildly, but the growth in the rate of return over the period was incredible.”

To check your use of the terms, try using “in” for “during” and “for” for “over.” (Sometimes, “in” and “for” may be used in place of “during” and “over.”)

Usage has made “over the long term” acceptable.