Harvesting should be done when the plant is at least 2-3 years old. In general, the grapes ripen at any time between the end of August until the end of October. It depends on the variety and the type of climate that is growing them.
To find out if your grapes are ready for harvest or not, try them out. If they have a sweet and pleasant taste, coséchalas. If not, then leave them for a few more days. Once the grapes begin to change their color it usually takes between 1 to 3 weeks to be properly ripe (how much time depends on the variety and how good the climate.) For example, green grapes usually become slightly translucent and their skins turn yellow once they are ripe.Ripped grapes also feel soft to the touch.Access time is all that matters.If you like the taste, then coséchalas.
@divina, maybe beacause of climate we have here in philippines the reason why it is not so sweet as I expected