There are 2 feeds one is permanent 12v the other only live with ignition on,
the permanent feed allows you to have the radio on with ignition off but only
for 20 min at a time the feed from the ignition should allow the radio to stay
on till you turn ign off, if it is acting like this it's normal if you are only getting
20 min with ignition on then you could have a blown fuse, I haven't a clue were
this would be on your vehicle but if you have a diagram or maybe there will
be a plan on the inside of the fuse box lid, there should be 2 one for each circuit.