Directly answering your question:
If a group of five Chinese citizens assemble in a Beijing park with a "Democracy Now" sign (or its equivalent in Chinese characters) for a few hours every Saturday, are they likely to be arrested within a year?
Honestly, it depends what you mean by "assemble".
If they are just hanging out every Sunday, and just happen to have a sign - or t-shirts - that say something like "Democracy Now", they will most likely not be arrested. In China they have all manner of silly T-Shirts. For example, "US Army" or USA Army" of "US Amy" or "USA Ary" or my favorite, "US RMY". People are wearing these everywhere. I meet a high level communist official wearing one of these funny shirts. My point is, about case 1, is that if they are not causing trouble, they will probably get no trouble either.
If they are chanting "Down with the state!" or "Free elections now" and they are harassing people to make sure their message is heard, and propagated, they will have trouble. However, in my experience, if it is limited to 5 people, it will not cause immediate arrest and imprisonment.
Likely, for case 2, the first few times one or two police will show up, ask them what they are doing, then tell them to disperse. After a few times, the police will report it to their superiors, who will find out where they work, or speak to their parents. Then their bosses or parents will tell them to knock it off. Generally bosses/parents are listened too, especially if family income is at stake. If none of this works, then there will be some sort of arrest. It is unlikely they will be sent to the countryside or to Xinjiang for re-education. A day in prison, and a friend bailing them out would be enough to stop it.
I recommend reading this post of mine over at expatriates SE about How to know what you can openly discuss in China.
As another anecdotal story about Chinese people and protests. A relative of mine lived in an older neighborhood in a million person city. The neighborhood is all single family homes; sort of a suburban feel. A wealthy real estate developer was planning to eminent domain the whole neighborhood, demolish it, then build a 20 story high rise investment grade residential property. Everyone in the neighborhood would have received a huge amount of money, and an equal area of condo to boot. However, the people in this neighborhood really liked their lifestyle, and they didn't want to sell. The law, of course, was on the side of the developer.
The people in the neighborhood had a choice - they could go to court and file an injunction, or they could protest by "city hall". They decided to try protesting first, and if that didn't work, go to court. Every morning, they arrived early to city hall plaza. They stood in front of city the whole work day hall protesting. They did this for about 5 days. It was about 20 people, and they were not causing trouble. Just protesting.
In the USA, it would be tear gas or something else unless they had a permit. In China, after 5 days the mayor realized they were serious. He came outside and asked them what the problem was. He was confused - "what do you mean, you don't want a big pile of money and a condo? what is wrong with you people?" was his response.
They told him, "no, we like our neighborhood. Please stop the demolition." The mayor decided to side with the people and stop demolition, and 7 years later, the crappy old single story neighborhood remains, surrounded by high value residential properties.
The point of this story is that for limited, reasonable protests, the Chinese govt can be fairly responsive and responsible. I want to point out, however, that had the mayor decided to demolish the neighborhood, and an injunction in court failed, that everyone would vacate and take the money.
So in summary, it depends how well the democracy protesters behave. Case 1, probably no trouble. If they are reasonable, they will not be bothered. Case 2, If they attempt to evangelize everyone, and are working take the CCP out of power, they will not be treated well.