I think the best time would be when you are both sure you're settled in life. When you both have security in work and with your home and just feel the time is right. That's when my late wife and I decided to try. Sadly it didn't work, but we were fortunate enough to adopt.
I think a couple will just know.

For us it just was like... welp there it is. Now we have baby. And we do want another one, but I'm SO waiting 2 years at least til we have another one. We had started right away though because we were afraid we couldn't conceive at all though, because of my medical problems.

You know it's the right time when your birth control pills decide that you are immune to them. xD No, but seriously. When you are mature enough to be capable of making life-changing decisions without second guessing yourself, feeling like shit afterwards, or you really just feel it in your gut. That woman's intuition or what-not. Having a steady secure job is a very very big plus. Both parents having steady secure jobs is a major bonus. Being able to keep a level head and figure out serious problems, that is a MUST.