Answer by taking a quote from the following article:

Kenneth William Kavanaugh (November 23, 1916 - January 25, 2007) was an American football player, coach, and scout. He played professionally in the National Football League (NFL) for the Chicago Bears as an end from 1940 to 1950, except for three seasons during which he served in World War II. He led the league in receiving touchdowns twice, and is a member of the NFL 1940s All-Decade Team. He is the Bears' all-time leader in receiving touchdowns, with 50.

After the war, he continued his career with the Bears. In 1945 he had 543 yards and six touchdowns, and in 1946 had 337 yards and five touchdowns. Three of his touchdowns in 1946 came in a 27-21 win over the Los Angeles Rams in week seven. The Bears defeated the New York Giants 24-14 in the 1946 NFL Championship Game, which gave Kavanaugh his third league championship with the team. The first touchdown of the game was a 21-yard pass from Luckman to Kavanaugh. After the season, he was named to the United Press All-NFL first team. Kavanaugh had his most productive season statistically in 1947. He had career highs with 32 receptions, 881 yards, and 13 touchdowns. He set a Bears record by recording a receiving touchdown in seven straight games, a streak that began with a three-touchdown game against the Boston Yanks in week six. His 13 receiving touchdowns led the league, and he earned his second straight first-team All-NFL selection from the United Press. Kavanaugh was named to his third straight All-NFL first team in 1948, and he again led the league in receiving touchdowns in 1949, with nine. His most productive single-game yardage performance came in his final season, in 1950 against the Yanks, as he caught eight passes for 177 yards and a touchdown. Kavanaugh spent a total of eight seasons in Chicago. He spent the majority of his career catching passes from quarterbacks Sid Luckman and Johnny Lujack. He remains the Bears' career leader in touchdown receptions, with 50. He also holds franchise records for highest career and single season yards-per-reception. His 13 touchdown receptions in 1947 is a single season Bears record he shares with Dick Gordon, who tied it in 1970. In 1969, Kavanaugh was voted by sportswriters to the NFL 1940s All-Decade Team. The Professional Football Researchers Association named Kavanaugh to the PRFA Hall of Very Good Class of 2009

how long was he with Chicago?
Kavanaugh spent a total of eight seasons in Chicago.