Valerie Trierweiler's chief of staff Patrice Biancone said yesterday she could remain a patient for another six or eight more days.
He said she suffered a "very strong emotional shock" after the affair was detailed in a French magazine last week and needs rest.
The scandal is likely to overshadow a major public appearance by Mr Hollande today and pollsters say it is likely to further damage his record-low popularity ratings.
The president is due to unveil plans to boost the flagging economy this afternoon but his news conference could be put in the shade by the ongoing controversy surrounding his private life.
He and Ms Trierweiler have lived together since 2007 and she occupies the so-called "madame wing" of the presidential palace.
She was admitted to hospital "for rest and some tests" last week after Closer magazine reported an affair involving the president and actress Julie Gayet, who it is claimed he visited at night on the back of a scooter.
Mr Hollande did not deny the report but protested at invasion of his privacy.
Conservative opposition leader Jean-Francois Cope has suggested the drama has damaged France's image abroad and political commentators have called on Mr Hollande to clarify Ms Trierweiler's role.
The twice-married Ms Trierweiler, 48, became Mr Hollande's partner after he left his university sweetheart and the mother of his four children, Segolene Royal, who later ran unsuccessfully for president in 2007.
Although they never married, Mr Hollande was Ms Royal's common-law partner of nearly 30 years and he strongly backed her political ambitions.
By the time he became president, however, in 2012, Ms Trierweiler, a journalist, was at his side and she has since grown into the role of First Lady.
She accompanies the president to official functions and manages affairs with her own staff of five in the Elysee Palace. Ms Trierweiler is said to be bossy and deeply unpopular with the public, the latter a trait she shares with Mr Hollande whom polls already suggest is the most unpopular president in a generation.
The latest poll - on Thursday a day before the latest scandal broke - indicated only 25% of French people trusted him.
The French media face much stricter privacy laws than in other countries but the tradition of secrecy over the private lives of public figures has been steadily eroded in recent years.
However, opinion polls suggest the French public appears to be averse to reporting politicians' private lives, although copies of Closer featuring the expose of Mr Holland's liaisons sold out across France.
French newspapers have also been focusing on the scene of Mr Holland's night-time encounters with Gayet, asking questions about the flat, near the Elysee Palace, where the pair apparently met.
The apartment was borrowed by Gayet from a friend, another actress, who has links to the Corsican mafia.
Opposition politician Mr Cope, leader of the centre-right UMP, said while he was "very attached to respect for privacy", the scandal was a "lesson in humility" for Mr Hollande.
He added: "All of this is disastrous for the image of the presidential function."
One report yesterday said Ms Trierweiler was "ready to forgive" Mr Hollande, depending on what he has to say about his "intentions" for their future.
The presidential couple were undergoing a "few days of reflection", the source said, adding Mr Hollande had informed his partner of the claims himself.
A friend of Ms Trierweiler was quoted as saying: "The news hit Valérie like a TGV hitting the buffers. She was completely stunned."