There is no hard and fast rule. There are however a few basic tips on general eating that we like to offer.
Only eat when you are hungry
Drinking water can sometimes make you feel less hungry, so always try that first.
Do not force food down
Do not eat a meal unless you have to
Do not finish the food on your plate unless you are still hungry (you will need to start eating mindfully)
Try fasting. It is like starving, but it is intentional and it works better than anything, if you can do it.