Wouldn't take it as the perfect answer but with any rechargeable device you're SUPPOSED to give it a full charge. However, every device I've ever had has come with a little bit of charge (normally about 50% of max) and I've used it straight away.
If the battery was supplied completely dead, then two or three minutes of charge should let you turn it on. If you are intending to start charging and using it, then as soon as you can turn it on, the charger should usually be able to supply enough power to power the phone, make a call AND charge the battery simultaneously.
However, i could be wrong.