How long should it take for an ipod to fully charge ?

About 3-4 hours.

In general, for a functioning battery, the longer it takes, the more capacity in your battery. When the battery is finally ready to die, it will seem to charge in a very short period of time.

Think about this: If you just stick dead battery on a charger (a resistor say 1 K-ohm acts like a dead battery) on a 5 volt charger when the current goes to 20 ma, the voltage across the resistor will be 5 volts, appearing to be fully charged. If you put a battery with lots of capacity in this same circuit, the battery will take the current and change the chemistry of the battery, and not show as 5 volts until the battery stops taking a charge, at which point it's fully charged.

Hope this helps.