I'm trying to make a part's decal appear every certain amount of seconds?

Assume script.Parent is where the decal resides in, Decal have a property called Transparency so fairly simple.

To create a endless loop you must use "while true do".

decal = script.Parent.Decal -- refers and sets a variable for the Decal, the decal will be named Decal of course.

while true do
    wait(120) -- 2 minute wait
    script.Parent.Decal.Transparency = 0
end

This is what you specified, but if you want it to always fade and come back again endlessly, where it becomes fully visible in a 2 minute wait this is a proper script:

decal = script.Parent.Decal

while true do
    for i = 1, 0, -0.00833333333 do
        decal.Transparency = i
    end
end

But if not that then you just want a regular script that instantly turns decal invisible and visible once again then this is the script:

decal = script.Parent.Decal

while true do
    script.Parent.Decal.Transparency = 1
    wait(120)
    script.Parent.Decal.Transparency = 0
end