Well, closest thing I can think of varies depending on several factors: how fast they are traveling daily, where they depart from, where they land, and where they are being stationed. I'd imagine that crossing the inner sea wouldn't take too long, it looks to be around roughly 40-30 miles wide in some areas, seeing as it's been reckoned that from Mournhold to Dagoth Ur is 250 miles. Also, depending on where they depart and land, there are several possibilities. If they were being sent to Fort Darius in Gnisis, they'd probably leave from a northern city like Blacklight. Possibly they'd have to go through Ebonheart since that is the main Imperial location in Vvardenfell, probably leaving from a more central location like Balfalls (a town on older maps and I believe mentioned in 2920, but there's no telling if it's still there) or Old Ebonheart (again, no telling if that location still exists within current lore).
Seeing as I'm currently killing time before going to work, I don't really have the time to work out the math... other people would most likely be able to provide more details concerning ship speed and troop movement.
EDIT: withing? well, at least that part is fixed.