it depends on the type of steel, ambient temperature, heat treating regime and geometry of the piece as to how critical time is; but as a general rule get a stress relieving temper (anything over 150C) as soon as possible. Once that first one is done, you can do your proper tempering at any time there after.
Most spring steels will be ok for hours or days (thought there is a chance of dropping it and thus breaking in the meantime), but I've watched a drawknife made from O1 bend and then snap whilst it sat on my bench waiting for the oven to come up to temperature! These days I have the tempering oven up to temperature (first tempering run at a lower than intended temperature such as 180-200C) before I even quench the blades, that way as soon as they are hard they can go in still warm