Answer by taking a quote from the following article:

Thierry Daniel Henry (French pronunciation:  [tjeRi aRi]; born 17 August 1977) is a retired French professional footballer who played as a forward and is the second assistant manager of the Belgium national team. He played for Monaco, Juventus, Barcelona, New York Red Bulls and spent eight years at Arsenal where he is the club's all-time record goalscorer. At international level he represented France and is his country's record goalscorer. Henry made his professional debut with Monaco in 1994.

In 1990, Monaco sent scout Arnold Catalano to watch Henry, then at the age of 13 in a match. Henry scored all six goals as his side won 6-0. Catalano asked him to join Monaco without even attending a trial first. Catalano requested that Henry complete a course at the elite Clairefontaine academy, and despite the director's reluctance to admit Henry due to his poor school results, he was allowed to complete the course and joined Arsene Wenger's Monaco as a youth player. Subsequently, Henry signed professional forms with Monaco, and made his professional debut on 31 August 1994, in a 2-0 loss against Nice. Although Wenger suspected that Henry should be deployed as a striker, he put Henry on the left wing because he believed that his pace, natural ball control and skill would be more effective against full backs than centre-backs.  After a tentative start to his Monaco career, Henry was named the French Young Footballer of the Year in 1996, and in the 1996-97 season, his solid performances helped the club win the Ligue 1 title. During the 1997-98 season, he was instrumental in leading his club to the UEFA Champions League semi-final, setting a French record by scoring seven goals in the competition. By his third season, he had received his first cap for the national team, and was part of the winning team in the 1998 FIFA World Cup. He continued to impress at his tenure with Monaco, and in his five seasons with the French club, the young winger scored 20 league goals in 105 appearances.  Henry left Monaco in January 1999, one year before his intimate and closest teammate David Trezeguet, and moved to Italian club Juventus for PS10.5 million. He played on the wing, but he was ineffective against the defensive discipline exhibited by teams in Serie A, struggling in a position that was uncharacteristic for him, scoring just three goals in 16 appearances.

How long was he at Monaco ?
Henry left Monaco in January 1999,