This really depends on the nature of the illness, if it is life shortening/threatening and what kind of impact it would have on the relationship itself and the new partner themselves.
I would be inclined to think that the earlier the better to tell in any case, I wouldnt blurt out on date 1 - I have 'long term illness', but I would try to bring the conversation casually round to it quite early on.
If it is something that could affect the relationship itself, like HIV for example, then I would be telling well before engaging in any practice that could risk transmission.
If it is something non communicable, but that your partner would need to know about in case their assistance is required (Im thinking perhaps epilepsy or something of that nature), then Id be telling fairly upfront.