you need it, so you take a guess with what you need now, and then tweak in the future it as needed. Think of a teeter totter with your assets on one side and your life insurance on the other. As your assets grow, you need less life insurance.
What I wouldn't do is get some humongous policy for 30 years and base it on what guess you need in 10 or 20 years. It is also a big cost when you have so little money when you are just starting out, so over insuring (versus needs) is like buying lottery tickets. There are lots of people that over insure, lots of people that under insure, but I think very few that hit the sweet spot.
Buy some before your wife gets pregnant with your first kid. As you add kids, you may need to add more, so buy some more, and so on, but make sure to think about the end game and when you won't need it anymore. Maybe you get some now, some more in five years and a little more in 10 years. Who knows how your life is going to turn, so why insure it on a guess?
There isn't a set formula - everyone is different. Most important thing is to think about it rather than just get some big long term policy without thinking it through.
Oh, and don't forget about long term disability. That is probably the most under insured thing in America, and a risk.