I'm trying to plan an area for a campaign to happen in (first time DM, but I have a few months to plan) and I don't know the scale of travel well. Looking at the Koronus Expanse, can Explorers cross that in one trip? Half of it? I don't know how far 5000 light years is in relation to 4 for that. Also, how easily could they leave the Koronus expanse or some such other area should they want to? Should I have other groups of planets prepared for my group to run into should they decide to go to the edge of the expanse and jump?

I tend to use as inspiration for the distances; while some of the planet locations are canonically wrong, it's good for a general estimate. 5 days travel through clear warp, more if you're trying to cut through Cinerus Maleficum or something silly like that. Just resolve it as a single Warp Navigation event unless the players want to play it safe, dropping out regularly to get a fix on their location.

Easy way: keep recording the time travels you give to the players in order to avoid inconsistencies. Then don't be much exact about the time it gets to go from here to there. After all, it's not unheard of ships that dissapear for millennia, or ships that arrive to destination before departing...

When GMing Rogue Trader, I kept track of time passing in and out of the warp during the campaign. Don't forget about the time dilation effects (mentioned in the navigator section). A week in the warp can be months outside of it. At one point, the navigator pc deliberately went for a slow time channel and passed a few *years* in real time (the crew wanted to avoid a major battle with a plausible excuse).