Update 15.6.1

This depends on your internet connection really - when it downloads the OS update. Since this is a new iPhone, if you are not restoring an old back-up, then it is even faster. Otherwise, the size of your back-up will also determine how long it takes to restore.

Depends on largely your ISP;internet service provider download speed.

Typically can be dependent on time of day, which day and local/global events such as Olympics, World Series, Soccer finals, etc.

With all of that said, most updates under 2Gb are less than 30 mins of actual download from internet if your ISP provides 200Mbs down, 20Mbs uploads speeds.

Then the update must verify and unpack update to actually install on system. This can take sometimes as long as the download.

Dont ruin your day by unplugging the power or interruption of install.