For how long can oil paint be worked into?
There is no simple answer, it depends on many factors like the temperature of where you live, sun and wind exposure, the amount and type of oil, the thickness of the paint, the color pigment itself. 

Obviously, the more you wait the harder it will be, but within a week should be fine. White spirit is not recommended it will attack and unsaturate the color, better use a painting medium. You also run the risk of violating lean over fat by using a variable quantity of white spirit. 

There are also mediums that delay the drying time, but the easiest way is to use oil as a medium.