Hi Ahmad.
Minimum 1 hour, but the further you go, the better it will be. From Sydney, West is the best bet
Also at the moment, the best parts of the milky way are rising later in the evening with Crux, Carina etc rising after 10pm.
Midnight/1am and beyond is when it really starts getting good, but it's not until April/May that the centre of the milky way is overhead at midnight.