"Earlier this year, Sam Stovall, U.S. equity strategist at S&P Global Market Intelligence, noted that the S&P 500 has a fairly good record of predicting election results. Since 1944, the incumbent person or party was reelected 82% of the time when the S&P 500 rose between July 31 and Oct. 31, according to Stovall. The only exceptions were in 1968 and 1980, when there were popular third-party candidates in the picture."
We'd need to know the number of times the S&P 500 rose between those dates to confirm the number, but the phrase 82% of the time means the number of times the incumbent was re-elected divided by the number of times the S&P 500 rose between those dates. When we make the generalizing statement, we use the singular, "of the time", consolidating the times into a whole and taking some fraction of it:
I go to this restaurant most of the time.
I go to several restaurants. This is the one I go to most often.
She took the train home 100 times last year. It was late leaving the station 25 times. It was late 25 percent of the time.
"25 percent" expresses the relevant fraction of the whole.