I would definitely not wait for long, but I wouldn't be to quick on it either.. I had my first yorkie living at home, and when he was 2-3 years old my mother bought another one because Eros was more mine than anyone else in our family, lol. But he was used to share his home and his people with other yorkies because we watched like 4 yorkies at the same time! It went very well to bring the second dog to our household, but I'd wish we got Emma (my mothers yorkie) a little bit sooner, maybe he wouldn't depend so much on me, and more playful with other dogs..
Me and Eros moved in with my fiancee and then came Molly the yorkie (4 months old now) and Eros is 5 years..
I don't think that it a certain age that fits better, I think it's more important to find a puppy that fills out and matches your first dog
But personally I would never buy another one before the first one is fully grown, remember the teenager period can be difficult and he can fast change personality!