When should I ask her to marry me?

Having children doesn't mean marriage would work for you. If you are going to rent an apartment do that first. So that you can see how you two with the babies do in your own place. See if you two have the same family values, expectations on social life, goals that won't take away from each other, what you think a wife role is and what she thinks a husband role is in a marriage. In other words, think deep before you leap.