I've left phones plugged in for months with no problems. (It happens in a shop, when you plug a non-working phone in and forget about it.) The fire hazard is the same whether it's plugged in for 5 minutes or 5 months - the charger stops charging the battery when it's fully charged. If it's plugged in for hours or days, and left on, the charger will kick in every so often to top the battery of, then shut itself off. (If the phone is off, it has to be plugged in for a week or two before the battery self-discharges enough for the charger to kick in.)