Hello Chapper: I see this is your first post here on PC. So... welcome to PsychCentral!
I hope you find the time you spend here to be of benefit.
I don't know if I really have any particular insight into the question you pose. My thinking I guess, with regard to how early in a relationship a person should disclose they have no particular desire for sex, is that this should be disclosed as early in the relationship as it is comfortable to disclose it. In other words, I certainly wouldn't just blurt it out at the first opportunity. But I would mention it as soon as it became comfortable to do so if that makes sense. The potential problem is that the longer you wait, the more entangled you may become with the person you're seeing & the more complicated it may become to bring the subject up... possibly to the point where you end up not disclosing it at all... or not disclosing it until it becomes really difficult to do so.
There's an analogy here I'd like to share. I've had some life-long gender identity issues. And, although I've never done anything about them, I've been interested in the experiences of people who transition from one gender to the other. One of the concerns that can arise is how early in the development of a relationship one discloses one has transitioned. If you disclose the information right away, it may eliminate the possibility of having a relationship develop at all. But, the longer you wait, the more difficult it may become... & the more severe might be the reaction. There aren't any hard-&-fast rules with regard to this. Individuals have to make their own decisions based on the dynamics of each relationship they encounter. And how they handle the question may vary from one relationship to another. I think the question of how soon to disclose you have no desire for sex is similar.
There are, as you can imagine, lots of articles in PsychCentral's archives on the subject of romantic relationships. Here are links to some that may be of some interest:
https://psychcentral.com/blog/why-he...ries-in-yours/
https://psychcentral.com/blog/14-tru...tic-true-love/
https://psychcentral.com/blog/how-do...from-intimacy/
https://psychcentral.com/blog/the-5-...-relationship/
https://psychcentral.com/lib/book-re...th-an-asexual/
I don't know, of course, if you're here simply seeking advice with regard to this particular concern or if you plan to hang in here with us.
However, should you be planning to continue on (we hope you do)... may I suggest you introduce yourself over on PC's New Member Introductions forum? Here's a link:
https://forums.psychcentral.com/new-...introductions/
There's a lot of support that can be available here on PC. The more you post, & reply to other members' posts, the more a part of the community you will become.
Plus there are the chat rooms where you'll be able to interact with other PC members in real time (once your first 5 posts have been reviewed & approved.) So please keep posting!
P.S. I don't know if you consider yourself to be asexual. But there is a website for asexual people I became aware of recently. here's a link:
https://asexuality.org/