How much money do I need to retire?

In order to reach my goal of retiring by age fifty I will need to define what it will take to retire. Theres a couple assumptions that I can take into account. First at the age of fifty I assume that I will have two children and be married. I also assume that I will have paid off my home and own it outright. Since I'll be retired I am going to assume that I'll have to pay my own health insurance.

I've done a quick rough budget and figured that $40,000 annually would be enough to live off of comfortably. I'm going to add a about 10% buffer and account for income taxes and round up a bit and I come with a rough retirement budget of $50,000 per year.

Goal: $50,000 / year income from investments and my house paid off by age fifty