Hm. Is there some sort of established average? Like, say, that travel between two nearby systems should take a few weeks, if everything goes well? Or is it all completely random?
BTW. I was wondering: when a ship travels through the Warp, what exactly its surroundings look like? I mean, is it flying through some some empty space filled with energy etc. - or is it rather flying over something more concrete, with the ground below etc.?
It's not as variable as other people are making it out to be - sure, there are a lot of instances where ships exit the warp at timings that are very odd, and this happens at a frequency that is probably completely beyond the safety parameters of our modern, enlightened society. But generally speaking, if you're getting on a warp vessel, you're going to end up at your location within reasonable time, give or take a few days/weeks. 40k has never been very absolute about warp travel times, but it's not only distance that matters, it's also 'vagaries in the warp' - navigators take into account eddying movements in that alternate dimension, avoiding the strange beasts (like warp whales) in there, while accounting for the strange way time flows there. But travelling is usually a matter of a few months, if memory serves correct from the various Black Library books. The Astronomicon serves as an 'anchor', a reference point for navigators so that they can, well, navigate in the warp; that's why it's so essential to keep it maintained. If I'm not wrong, there was an incident where the Golden Throne went 'blip' and multitudes of ships were lost in the warp.
But normally speaking, if you board a ship in 40k, you're going to get to your destination. Instances of warp-related failures are miniscule in comparison to the likelihood of being attacked by pirates, raiders, Chaos warbands, and so on - which are incidentally, not common enough to prevent a strong interstellar trade system continuing to be the economic base of the Imperium.
Looking outside of a ship is afaik not allowed, and not very wise; it is a daemon dimension after all. By right a ship has Gellar field to protect itself from the denizens of the warp; this has to be constantly maintained and kept up while in the warp. Many ships operate with arcane, forgotten, crumbling tech, so it's not unheard off that the Gellar field has collapsed while a ship has been in the warp, dooming the vessel to be overtaken by daemons. Prayer, apparently, also works. My Imperial Infantryman's Uplifting Primer said so.