"I scout ahead, keeping to the shadows as best I can in case anything is watching."
In this instance the Rogue is potentially unaware of any observers and must use cover and concealment/shadows as best she can. Ironically, the best route through a large room might well be straight through the middle, out of range of the torch sconces on the wall. In the interests of speedy play, it might be tempting to assume that the Rogue will have the expertise to minimize her chances of being spotted even if the player does not... but the player in question will have to trust the DM when it comes to potentially falling foul of concealed ambushes or traps, because it'll be the DM that ultimately decides what route she took in this instance.
You could describe the room and have her plot a simple route ("I stay towards the center, moving through the overturned pews..."), or get a routine from the Rogue beforehand ("In general I will stay as close to the left of the room as possible, but out of bright light..."), or you may be prepared to plot a battlemat for every room with light-source overlays (not as daunting as it sounds, especially in compact dungeons where every room is designed as a challenge). All of these will slow down play to some extent but the potential result is to get the Rogue thinking strategically about her movement, ideally without bogging the game down too much. You may suddenly find you're encouraging real tactical thinking from those gamers who've played Thief or Splinter Cell ("My wizard friend uses mage hand on this flask of water to extinguish the torches on the eastern wall...").
Things would be easy if the Rogue could point at a villain and say "I'm hiding from him..." but in this case she's moving stealthily to avoid being seen by someone she doesn't know but assumes might be there. When the time comes for something to actually Spot her, decisions will need to be made both on how many Spot checks the observer receives, and the relative location of the Rogue each time. Now you could use Spot's '-1 per 10 feet' rule to calculate Spot distances I suppose, but the simplest solution is just to get the Rogue to roll a Hide check for each 'round' that she needs to spend to get across the gap (even though, out of combat, rounds don't technically exist). If you wanted things even simpler, you could just ask for one roll to cover it all, but then the Rogue is no more likely to be seen crossing 100 feet as she is crossing 20 feet, and that's just not cricket.