If my dogs were 'just' pets like they used to be, I'd say a year and a half to two years probably. But now that I'm getting more into sports, I can't really picture adding another until Journey is at least three. At least. Probably more like four or five. Entry fees cost, training can be costly and is time consuming, and I just feel like I'd be being unfair to Journey in that regard and others to add another dog before Journey turns three, and like I said, even then might be too soon as much as I'd love another.
The age difference between Keira and Dance is 8 months (it worked just fine because of their personalities, but I wouldn't necessarily recommend it always) and the age difference between Keira and Ripley is just over a year (16mths I think). While everything was pretty smooth, I think it would have been better to wait longer between dogs. The four year age gap between Journ and Ripley is a good one for me, and I think my goal from here on out will be 4-5 years between dogs.
But really, I think much of it is dog dependent too. It was easy to add another young dog with Dance because Dance was never like having a puppy at all. We probably should have waited between the two Dobermans just to give Keira some more maturity and time, but she was mentally pretty past the puppy stage by the time Ripley came home so it worked out fine. And alternatively, for as easy as Journey is, she is pretty demanding as far as different needs go, and I really can't imagine adding another young dog/puppy at this point while keeping life fair for Journey. We have quite a bit of fun and learning to do together before I can realistically consider another.
ETA: I forgot to say also, I don't think I'd ever want more than 5 years between. I like to have an adult dog that still wants to interact and play with a puppy/young dog. Dance (almost 7yrs) and Keira (6yrs) have pretty much zero interest in other dogs or playing with them at this point. I like that Ripley is still puppy enough at 4 1/2 to play with Journey regularly... although I think it helps that he's a boy, and therefore probably will never grow up and become "too good for stuff" like my grouchy girls haha.