Hi,
I think you'll get differing opinions on this one, based on everybody's experience. A mom of a colic baby will probably say that never is a good time!
Sounds like your baby must be fairly manageable, otherwise you wouldn't have thought about another one. I think the important thing is that you and your partner must be able to cope logistically, mentally and financially. Then it doesn't matter what the age gap is. Every baby is a bit of a lucky packet, you may cope, or not. But that's true no matter the age gap, a new baby is always an adjustment.
Personally, I found that my daughter was the most challenging at 18 months. She was completely mobile and going into everything, she needed supervision more than at any other time. I know for a fact that I couldn't have had a new born at the same point in time.
She's turning 3 soon and will have a little baby brother shortly after, being a bit older has its own difficulties, possessiveness, etc. Still, in our situation it's better than having 2 kids under 2, that would've been a disaster in our household