It seemed like days and they were so comfortable like they were renting the place. I know I would smell fried chicken that was freshly prepared in my house so either that was a summer home or the owners were oblivious. I would've tried to go back there instead of that boiler room. Just saying
Zina ( The Original Princess)
They were there while the family was away at their summer beach house. Which depending on how far into summer the movie started at (which was never clear) it could have been two months or a few weeks until the family came back. One scene had Hannah finding pictures of the family at their beach house and the calendar in the kitchen stated when they would be back, which Hannah and Tahir planned on leaving before then, but obviously as we saw, a house keeper came in before that date to get the house ready for the owners return. So they had free reign of the place until that point.
That apartment would have definitely been better than the boiler room, but in the winter would have been occupied. (As even families with winter ski houses tend to only visit those on holiday or weekends as the kids are in school during the week in winter, and that would be the same for most houses in general, while there are many unoccupied pied-à-terre's in the city finding one that is both unlocked (they lucked out with that apartment in summer because they happened to be up on the terrace when they were running away and the terrace door was unlocked.) and currently unoccupied would be a challenge and very risky as you'd have to enter the place to really know.
The fact is, most homeless people don't shack up in other people's apartments while they are on vacation, there are many cases throughout the world of squatters moving into abandoned buildings (look up Tower of David in Caracas Venezuela for a fascinating example) and seeking temporary shelter in a place like a boiler room is far more likely than in a multimillion dollar apartment. Paul Bettany himself has even said that he only added the apartment scenes in because he was told that no one would be interested in his love story about homeless people because as one of his agents so crassly put it "no one wants to see homeless people kiss" so he added in a "goldielocks" segment where they get to essentially play dress up during the part of the movie where they fall in love in order to draw the audience's empathy, which makes sense, as most love stories you see on screen feature relatively affluent people.