Hmm this is a good question but I have a little insight into this from my own personal experience – when I met my partner they were only 12 weeks out of a 6 year relationship (!) and it was the same concern I had.
My advice is that it’s different for everyone and starting it slow and casual is a good way to do things, unless you want something else – if you are ready for a committed relationship then maybe set a bit of a timeline i.e. “I am happy to take things nice and slow but you know I want a relationship so can we have honest communication about how we are going”
Asking a person why they broke up with someone or how their relationship ended will give you some more information – if it was really sudden or they got their heart broken then maybe I would consider holding off. If it was a mutual and respectful breakup ‘we both identified that we wanted different things / grew apart / fell out of love’ then it sounds like they are more emotionally aware.
Again, everyone is different and so I think we expect the first relationship post break up to maybe be a rebound – especially it seems that the first people you will have sex with after normally are (in my experience) – but in the case of my relationship it was different and though I had doubts that it was too early for my partner I quickly realised that I didn’t have anything to worry about because we were able to talk it through and it because obvious that they had already recovered from the relationship before it ended if that makes sense.