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

The "Wait()" is in seconds

so you are setting the visiblity to true then waiting 15 seconds then waiting 1800 seconds