Depends on the Queen. Some turned a blind eye to it, others were less tolerant.

For example Philippa of Hainault, who was Queen of England as wife of Edward III, seems to have been one of the ones to turn a blind eye. She and Edward appear to have had a very supportive, loving marriage. They had thirteen children together and Philippa travelled to Europe with Edward on several occasions to support him. He had a collection of mistresses over the years, but none of them were ever in a position to have Philippa marginalised. The only exception to that is Alice Perrers, Edward's last mistress, who had an unusual amount of influence over him. Possibly because by this point Philippa was starting to become ill herself, and was less able to spend as much time by Edward's side.

In comparison you have Juana, an Infanta of Castile by birth and Duchess of Burgundy by marriage. She was notorious for flying in to jealous rages over the affairs of her husband Philip, although some of those stories may have been exaggerated in an attempt to show her as mentally unstable. Juana's mother Isabella was another "turn a blind eye" Queen, and Juana's sister Catherine of Aragon is also known for tolerating Henry VIII's mistresses to a certain extent (she was reportedly less tolerant over the way Henry favoured his illegitimate son by Bessie Blount), until Anne Boleyn came along.

It's often a combination of the type of woman the Queen was, and the kind of relationship the King had with both his wife and mistress. For some Kings their mistress was more there for physical release, especially when the Queen was unavailable due to pregnancy or illness. They still had a close relationship to their spouse, and the Queen still retained her influence. But such marriages weren't always guaranteed, especially as they were arranged for political reasons. A King who wasn't in love with his wife and just saw her as a breeding machine, or who was otherwise a weak character and susceptible to influence, was more likely to have wife less tolerant of a mistress. Especially if the mistress then used her position to influence the King.

Other tolerant wives include Catherine Braganza (wife of Charles II, she wasn't happy about it but she learned to ignore it), and Maria Theresa of Spain (wife of King Louis XIV of France, even went to watch one of her rivals become a Nun). I should also point out that there is no evidence that Eleanor of Aquitaine had her great rival, Rosamund Clifford, murdered. Especially since Eleanor herself was in disgrace and imprisoned at the time.

Sources; Philippa of Hainault; There's a very good chapter on her in "Queens Consort" by Lisa Hilton. The Plantagents by Dan Jones Joan of Kent by Penny Lawne, and Katherine Sywnford by Alison Weir, both have references to Philippa of Hainault but off the top of my head I can't recall how much they discuss the mistress issue.

Juana of Castile; Sister Queens by Julia Fox (looks at the lives of both Juana and her sister Katherine of Aragon) I think Thomas Penn's "The Winter King" work on Henry VII might also mention Juana as Henry considered marrying her after his first wife Elizabeth of York died, but I can't find my husband's copy on our bookshelf to double check this.

For Maria Theresa of Spain's reactions to her husband's infidelities I can highly recommend Antonia Fraser's work "Love and Louis XIV", while her biography on Charles II also details Catherine of Braganza's original attitude towards his mistresses (hysterical crying) and her gradual change (tolerance) in order to keep Charles' love.