I have been on here for about 6 months. I post a hub about every other week. I read and comment on people's hubs daily. I answer a couple of questions daily. I rarely get on forums.
If you write a quality hub every week or two, with few grammar or spelling errors, your hub scores will be in the 70's and 80's. Those fluctuate daily with the number of readers you get. Participating in the community (answering questions, forums, hub hopping) will raise your hubber score 1-3 points each. Reading and commenting on other people's hubs will get you followers, which will bring you readers. Those help your hub scores rise. Holding a hub for 24-48 hours after you're done with it, will give you a slightly higher hub score to start out with, which will help keep your hubber score from fluctuating as much. It will drop slightly every time you publish a new one until you get a large number of hubs to average.
How long it will take for you to get a hubber score in the 80's and 90's is up to how much work you want to put into it. Getting the elusive 100 is not as easy. Also getting it and keeping it are two different things. That could take years to get. Keeping it is impossible. I only know of a handful that keep them for more than a day or two. When we do get them, we watch them suspiciously, waiting for it to drop. It is almost a joke.
Most people never worry about the hubber score fluctuating because they will. They will also drop, seemingly for no reason. Just do your best to keep your work at a quality level and be out there in the community to attract readers and you will do well. Some people will make fun of you for worrying about it, so don't bother.