In the GATE anime (ep. 3-4), Itami goes to Coda village, then returns to Arnus Hill with the children and the left-behind refugees.
From the time he left to when he returns, the JSDF camp went from a tent park to a fortified pentagonal structure with buildings.
Now XXI century building tech can make a 57-floor-tall building in 19 days, but we are talking about reinforced military-grade structures with materials being shipped through a portal, so YMMV.
The anime never tells how many days Itami was away during his Coda Village reconaissance sortie.
Does the light novel tell of the passing of time during Itami's mission?