for some reason it is only visible and after 15 seconds it stays visible. Help?

I am not sure what you are trying to achieve, please be more specific next time, but I can explain it for you.

while true do
    game.StarterGui.ScreenGui.Frame.Visible = true --//Setting it as visible
    wait(15) --//Wait 15 seconds
    game.StarterGui.ScreenGui.Frame.Visible = false --//Setting it as not visible. 
    wait(1800)
    game.StarterGui.ScreenGui.Frame.Visible = true --//Setting it as visible
    wait(15)
    game.StarterGui.ScreenGui.Frame.Visible = false --//Setting it as not visible. 
    wait(1800)
end

game.StarterGui.ScreenGui.Frame.Visible = false

That will make the frame not visible.

Sorry if I didn't answer it, i barely understood it.

EDIT: Realized the script wont work because your calling ScreenGui.

Do this:

while true do
    script.Parent.Frame.Visible = true --//Setting it as visible
    wait(15) --//Wait 15 seconds
    script.Parent.Frame.Visible = false --//Setting it as not visible. 
    wait(1800)
    script.Parent.Frame.Visible = true --//Setting it as visible
    wait(15)
    script.Parent.Frame.Visible = false --//Setting it as not visible. 
    wait(1800)
end

and put the script in the ScreenGui