How do I make each day last longer?

Much like Fearme987 said, just change the number inside the parenthesis after "wait"

Example: Wait(60)

That will make it wait 60 seconds before actually changing the time of day, I also suggest checking out the wiki page below on how to create a day/night script. It shows you how to loop it, make it shorter, and even make things happen at certain times of day!

http://wiki.roblox.com/index.php?title=Making_a_Day/Night_Cycle

Or you could if u wanna go by hour then do this

minutesAfterMidnight = 0
while true do
    minutesAfterMidnight = minutesAfterMidnight + 60
    game.Lighting:SetMinutesAfterMidnight(minutesAfterMidnight)
    wait(60) -- Where 1 hour(in game)  = 1 minute(real life)
end

if you want time to change every minute then do this

minutesAfterMidnight = 0
while true do
    minutesAfterMidnight = minutesAfterMidnight + 1
    game.Lighting:SetMinutesAfterMidnight(minutesAfterMidnight)
    wait(1) -- Where 1 minute (in game)  = 1 second (real life)
end