I don't think there is an appropriate age gap between siblings is there? It all depends on you and your families circumstances and personal choices.
I've found most folk prefer to have children close together in age. I however went the total opposite. There is a 16 year age gap between my two and that for me worked very well. I was able to devote 100% of me to my first one, and then when she was more or less independant of me, I had my youngest and again, I'm able to devote 100% of me to her.