Your question is worded a little oddly so I'm not sure which car your referring to. The long and short of it however is that they can stay on until the burn out. Long glow session will also reduce the life of the plug. If they were left on for long enough to warm the block I would pull each one out and make sure that they are still functioning.
On a side note I believe the stock relay limits the plugs to 30 seconds of glow time before they switch off.