Question: For how many seasons did the series in which Courtney Thorne-Smith played Lyndsey McElroy run ?

Context paragraphs:
Summer School (1987 film): Summer School is a 1987 comedy film from Paramount Pictures, produced by George Shapiro and Howard West, directed by Carl Reiner, that stars Mark Harmon as a high school gym teacher who is forced to teach a remedial English class during the summer.  The film co-stars Kirstie Alley and Courtney Thorne-Smith.  The original music score was composed by Danny Elfman.

The Lovemaster (film): The Lovemaster is a 1997 American comedy film directed by Michael Goldberg and starring Craig Shoemaker, Courtney Thorne-Smith, George Wendt and Richard Singer.

Breach of Conduct: Breach of Conduct is a 1994 television film starring Peter Coyote and Courtney Thorne-Smith.

Courtney Thorne-Smith: Courtney Thorne-Smith (born November 8, 1967) is an American actress.  She is best known for her roles as Alison Parker on "Melrose Place", Georgia Thomas on "Ally McBeal", Cheryl in "According to Jim", and her recurring role on "Two and a Half Men" as Lyndsey McElroy.

Day by Day (TV series): Day by Day is an American television sitcom aired on NBC.  The series ran from February 29, 1988 to June 25, 1989.  It stars Douglas Sheehan, Linda Kelsey, Christopher Daniel Barnes, Julia Louis-Dreyfus, Courtney Thorne-Smith, and Thora Birch in her television debut role.

Two and a Half Men: Two and a Half Men is an American television sitcom that originally aired on CBS for twelve seasons from September 22, 2003 to February 19, 2015.  Originally starring Charlie Sheen, Jon Cryer, and Angus T. Jones, the series was about a hedonistic jingle writer, Charlie Harper; his uptight brother Alan; and Alan's troublesome son Jake.  After Alan divorces, he moves with his son to share Charlie's beachfront Malibu house and complicates Charlie's freewheeling life.

Welcome to 18: Welcome to 18 (alternative title: Summer Release) is a 1986 American coming of age comedy-drama film starring Mariska Hargitay, Courtney Thorne-Smith and JoAnn Willette.  The film was directed by Terry Carr who also wrote the screenplay with Judith Sherman Wolin.

Side Out: Side Out is a 1990 American comedy-drama sports film about a beach volleyball competition, featuring C. Thomas Howell, Peter Horton, Harley Jane Kozak and Courtney Thorne-Smith.  It was the last film from Aurora Productions due to many flops.  The film was directed by Peter Israelson.

Lucas (film): is a 1986 American teen tragicomedy film directed by David Seltzer and starring Corey Haim, Kerri Green, Charlie Sheen and Courtney Thorne-Smith.  The film was Winona Ryder's screen debut.

Chairman of the Board (film): Chairman of the Board is a 1998 comedy film directed by Alex Zamm, starring Courtney Thorne-Smith and Carrot Top.  In the film, a surfer and inventor named Edison inherits and runs a billionaire's company.  It was poorly received by both critics and audiences and is regarded as one of the worst films ever made.

Supporting paragraphs: Courtney Thorne-Smith Two and a Half Men
Answer: twelve seasons