"We are coming in seven hours."
Translation:Vi kommer om sju timer.
Prepositions are used differently when it's about time. If you said "Vi kommer i sju timer", that would mean you would be arriving constantly for seven hours.
"Vi blir i sju timer" = "We're staying for seven hours" (i = for)
"Om fire timer slår klokken fire" = "In four hours the clock strikes four" (om = in)