The time to takes to install a solar array depends on system complexity, type of installation and system size, among other things.
A typical residential roof array takes approximately 1-2 days to install. Array size, roof slope, disconnect logistics (Will a trench need dug?) and uncooperative weather can add to this time.
If trenches will need to be dug and/or if concrete piers will need to be poured (most often with pole mounts, some canopies and sometimes with ground mounts) or if the solar will be installed on a custom metal frame; these things can add to the length of the installation time.
Note, that most systems require interconnection and municipal inspections before they can be commissioned or turned on.