"How long do I socialize my pup?" "How do I know when I've socialized enough?" Ahh, questions that should perhaps be asked more often. Because socializing is so important.
Yes, young puppies need to have great experiences with different people, dogs, environments, noises, and surfaces (it's a long list that I can give you), but your job really isn't done ever. Of course your older puppy still needs to see a woman with a walker and to receive treats at the vet and meet new dogs. And your adult dog does, too. Socializing never really stops. It doesn't have to be as constant as with puppies under four months old, but if you don't keep the new experiences coming, your dog could become a wallflower.
There are adult dogs who have had bad experiences with new things or just not had enough socialization when pups, and those dogs may feel very uncomfortable being around children or skateboards or the ocean. It's not easy to get some dogs out and about. But a trainer can help. Not every fear needs to be faced, but we have to be careful not to cloister a dog because of difficulty with a certain issue. A little dog may have a problem relaxing around larger dogs, but he should still get to hang out at a cafe with you. Your poodle may really dislike kids, so she doesn't want to watch the soccer game in the field up the street--but she would enjoy taking a walk around the park.
Keep your dog active and provide the mental stimulation she needs. Agility class or obedience or nose work or just finding breakfast kibble around the living room! Keep life interesting for your so interesting doggie!