Apollo is one year old (possibly older) and he NEVER EVER barks. The only time he barks is when something is wrong (like someone is sneaking around outside or knocking on our windows, some dogs aren't barkers (I LOVE that about apollo)
The first time I heard Nala bark was at herself in the mirror the second or third day we brought her home (8 weeks) since then she really only barks when she plays with other dogs, she gets in her play position with her butt up in the air and bark at the other dogs if they are ignoring her.
Ellie is 17 weeks, and she's been barking for a while. I'm guessing since 12-14 weeks? She sometimes barks when she wants to come inside, or when I'm getting her food ready, or when I tell her "NO!". I'd say your lucky to not have barking!
Labs are not generally a vocal breed. Think about it...a vocal breed that is constantly barking would make a useless gundog. Think of it as a blessing. I have done pet sitting for many barky dogs and it was enough to drive me barking mad.