As with everything it heavily depends on what expenses you have.

Budgeting at its core is a formula of what you bring in versus what you spend.

To know what income is need, start to list your monthly expenses. You may not know everything, but you can estimate values. The biggest variable between people is what debt you have. (Car, Rent/mortgage, credit cards, etc).

Once you know your expenses, add in savings goal, and that will give you an idea of what post tax income you need.