Wait until that dog is satisfactorily trained - not finished, because training doesn't, but at least with good, reliable, housebreaking and basic manners. Definitely wait until you are through the 'teenaged months' of puppy hood. Puppies are hard. Two puppies are three times as hard. A puppy and a well behaved dog is half as hard. (Ie: Dogs teach each other, too. A well behaved dog will help you. Another rowdy puppy will not.)
I have a 6 month old pup and I can't imagine raising another puppy ever, and I don't even have a difficult pup!! (he's housebroken and generally fairly pleasant) If I was going to add another, I would definitely wait until Hammie had settled down a good bit, and was rather reliably trained. For example, he still doesn't have run of the house because he can't be trusted not to chew on stuff or dig stuff up or pull stuff apart. That and I think the cats might try to kill him.
Are there any resources in your area for finding play mates for your dog? Mine goes to at least one play group a week (there's a second we were going to but his current obedience class is at the same time, so we'll resume when that ends), plus we meet up with some locals at the park on Sunday mornings, plus he has class, and he has a few doggie friends who come over to play. I actually placed an ad on Craigslist looking for friends for him and met a very nice little pup that he loves running around with!