My personal experience:
By around 6 months I was starting to stress about this, but a few things helped me put it into perspective. First, I wanted to breastfeed till at least 12 months and thought I would have to wean to go back to IVF. Second, I ideally wanted a 2-3 year age gap between babies. Third, I was getting no sleep and really didn't have the energy for the moment anyway. And finally, when I really thought about it, I just couldn't face IVF again for a while yet.
So from that I decided to wait till DS was 12 months and then think about it. 12 months came and we still weren't ready. from 16 months my cycles returned and we started ttc naturally, with a plan to return to IVF when DS was 2, which is what we did.

So, maybe think about where you are now (are you well rested? All recovered from the birth? for eg) and what sort of minimum age gap you'd like?