I have a lamp with a strong watt light bulb to keep my babies warm but I also like to add a hot water bottle for extra warmth. JUst wondering how long does it take for them to go cold (the hot water bottles) so I can change it again?? Thanks I guessed about 4 hours but does anyone know?? (I cover the hot water bottle in an old tshirt so i guess it stays warm longer!?!!? Thanks!!