It's not so much about the time but rather the direction the new relationship seems to be heading in.
If there is NO (mutual romantic interest) there is no need to run down all of your aliments. Should you become best friends and decide to confide in them down the road that's the normal.
However if it's clear that this person is harboring "romantic feelings" towards you after having spent some time with them then you should let them know by explaining you have some health issues before things go too far emotionally.
I would start off by saying: "I want to make sure you are aware of some things about me before things progress too much between us. Awhile back I learned I was....etc"
Then I would tell them what I have done to learn to cope with it and not have it destroy every area of my life.
Afterwards I would tell them that I wouldn't blame them if they decided that I wasn't "the one" for them or chose to be "platonic friends".
Essentially leave the ball in their court to decide whether they want to explore the relationship further. An impotent man may come across a woman who has no interest in sex and is simply looking for companionship. Age, timing, desire are often determining factors.
Having said that I don't believe your health issues should be automatic "first date" conversation. Most first dates are (only dates)!
It's not necessary to walk around with a rubber stamp of "impotence" on your forehead. Every relationship starts off with both people hiding behind "brick walls" of defense and over time each person removes one brick at a time as they begin to (trust) the other.
One reveals privacy concerns based upon (their) comfort level.
You're under no obligation to take out a full page ad in newspapers or make announcements about your health to everyone you say hello to or share a meal with.
Nevertheless the goal is not to mislead anyone.
If a woman has made it known she wants to give birth to children you have two options. Reveal you're impotent or tell her early on you have no desire to have children. Either way (she) is not "the one".