Hi Jan and welcome to Yorkie Talk. If there is one thing I have learned both from my own dogs and reading other people's stories here, no two dogs are alike. Some catch on quickly, others not so much.
I think it's great that you are doing your research before purchasing. But I'm going to have to give you bad news. ANY dog can be hard to housebreak.
Best case senario, you get a dog from a breeder that has already started training before you bring your dog home. Usually that makes it a little easier. But again, these guys all have such different personalities there's really no way any one can tell you how long it will take to train a particular dog.
One reason that I read that small dogs are so hard to housebreak is, since their accidents are so "tiny" in comparison to larger breeds, people tend to overlook them more than they would in another case. But then get upset when the puppy is older and is making bigger messes.
I've had my puppy, Tia for almost a month now and she is not even close to completely potty trained. She is 4 months old and has accidents everyday. I bought a good carpet cleaner, a spray that gets rid of the scent, and clean my carpets once a week. I'm very consistant with her, she just hasn't "gotten" it yet. Compared to a Yorkie I had as a child, she is moving very slowly. THAT Yorkie "got" it right away and didn't have ANY accidents after a month. Same training method, different dog.
There's always the chance that it will take a while to potty train a puppy, but usually once it clicks for them, it's there forever. There may be the occasional accident, but usually that isn't their fault.
If potty training is a huge issue for you, you might want to consider adopting an older dog from either a rescue group or a shelter. My older dog, Sandy is a rescue and came to me completely potty trained. The only accidents she has had is when she has been sick. So, that may be another way to go.
Whew - hope this was at least a little helpful to you
Good luck!