How long should you date someone (or at what point of a relationship) before a discussion on marriage? How to tell if your partner is ready for this discussion?

My bf (29M) and I (27F) have been together for 16 months. Early in our relationship, he told me that his ultimate goal is to get married and have a kid. It’s just to make sure we both look for the same thing before we emotionally invest in each other. We agree to get together and see how it goes.

I start to think that he’s the guy I want to spend my life with at the 12-month mark and I’ve been giving it a lot of thoughts these few months. I admit it’s a bit “quick” for me to decide but I’ve been through a rough patch and big change and I am assured of his importance in my life during those time.

I hate to put a timeline to a relationship but I have to be realistic if I want to have a kid during the prime age of pregnancy (before 32). I’m going to turn 27 soon so the thought of marriage crosses my mind often recently.

We’re both at a good position in terms of career and financial resources. It’s just I don’t know whether he thinks I am a partner for life. He’s also the most indecisive person I’ve ever met so I really don’t know what to do.

Tl;dr: So what point of relationship is a good time to ask your partner whether he/she wants to marry you? How long should you date before this discussion? How do you approach the subject without making it like an ultimatum? I don’t mean a proposal or a big plan. Just want to know if this relationship brings me anything that’s lifelong.