Well, there's eight months between Maddie and Molly, and 22 months between Molly and Mikki and they all get on really well. A few teething problems for a few days with the youngest two, but Maddie has accepted both of them with no problems. And they are all girls :)
Personally at least 18 months, preferably 2 years, depending on how well behaved your first dog is. I presume you are getting a gsd (from your username) which is a big strong breed which i believe can take a while to mature. If your first dog has reached what you feel to be an acceptable level of training and you feel you can cope with 2, then go ahead. Do be aware that the first dog can regress back to puppyhood slightly too.
I also would recommend 18m to 2yrs. my dogs range from 3y8m 2y3m and 6m 3wks .....recently had one of my litter puppies back for a holiday two puppies together ....totally deaf!! :)
beleve it or not i have a Leo, when i first joined CD i had a GSD hence the user name. I know you have two very close together Dee, dont know where to have another bitch or a dog, Storm is the first bitch i have ever owned and as missie and leomad know she is very "trying" in the eating department although in every other way she is a dream. The earliest i would concider a pup would be july which would make her 15 months old