How much you need to earn before marriage depends on several factors eg where you live, the standard you hope to maintain, distance of your house to office, how soon do you want to starting having children, do you have a car etc
Since there are so many variables, it will be difficult to give you a figure. Nevertheless, this is a scientific approach to it.
Write down your current monthly expenditures, adjust for rent and other things which may change after marriage.your partner too should do same. Both of you should harmonise your expenditures. This will give you a rough estimate of what you will need per month. Compare this with what both of you currently earn, it will show you where you are financially.
Will you need to reduce your standard? Will you need to get a cheaper accommodation? You are in a better position to know if your current earnings can sustain a new family or not.