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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? How long are they likely to stay in jail?

I have a friend from China who claims that they will be put in jail for more than one year (even if the "group" was just one person instead of five!), but I find that hard to believe. These people are just promoting democracy for their country. I know that I am biased by a Western view of freedom, but I also concede that assemblies with free speech can lead to uprisings which disrupt peace.

Assuming your question is to be interpreted as "show me examples", what happened more recently that's reasonably relevant to a scenario like you propose was Wukan, in which local anti-corruption and to a certain extent pro-[local]-democracy activists were eventually suppressed:

During the 2011 protests, Zhuang helped barricade the coastal hamlet of 15,000 people against battalions of riot police. He spoke by telephone with Reuters from New York, where he is now in exile after leaving Wukan in 2014. [...]

A months-long insurrection against local authorities and riot police put a global media spotlight on Wukan, leading to a rare populist victory in Communist China when the provincial authorities eventually backed down. [...] a free election was allowed that saw all seven of the protest leaders voted into public office.

This village committee soon came under pressure from allies of the old leadership seeking retribution, they said. Over the course of the next few years most of the protest organizers left public office, including their leader, Lin Zuluan, who was jailed for corruption last summer.

The protests last year erupted after villagers demanded Lin’s release. Many villagers say the charges were concocted and that a confession by Lin on state television was forced.

The protests ran for several months and were quelled by hundreds of riot police firing rubber bullets, hurling tear gas and beating up villagers with batons, victims and witnesses told Reuters at the time.

Last December [i.e. of 2016], the People’s Court of nearby Haifeng town, which oversees Wukan, sentenced nine villagers to jail terms ranging from two to nine years for a number of charges including illegal assembly, disrupting traffic and spreading false information, according to a notice on its website. [...]

One villager, Zhang Bingchai, was jailed for two years for allegedly spreading false information, the court said. Two acquaintances who knew him said he had spoken about the situation in Wukan, to outsiders on his mobile phone and had posted some images of the crackdown.

Zhuang said his 67-year-old father, Zhuang Songkun, was jailed for three years on a charge of gathering a crowd to block traffic during the protests.

The methods were not as drastic as in Tiananmen, so to more localized events the Chinese authorities respond somewhat differently nowadays.

For more see Wikipedia article on the 2011 protests at least 54 dissidents were arrested in the aftermath, some just for expressing sympathy with the protesters.

Also of some relevance are the "preventative" measures that Chinese authorities apply to known dissidents (often after they are released from jail), including "touristing" them under police escort.

In 1989, a large number of Chinese citizens did exactly this on Tiananmen Square.

The reaction by the Chinese govenment was to declare martial law and order the army to attack the protesters with lethal force. The military complied and killed several hundred to several thousand protesters (estimations differ a lot depending on source).

Now this was 30 years ago, and we were talking about a large scale protest, not just a small group of 5 people. But the Chinese government hasn't become much more open to criticism since then. Political activists with opinions contrary to the party position are frequently harrassed and arrested. The crime protesters usually get accused of is "Inciting subversion of state power". The maximum penalty is 5 years, even more for ring leaders.

Chinese citizens who publicly divert from the party line also face economic and social isolation through the Social Credit system.

So yes, the concerns of your Chinese friends are entirely justified.

China claims it protects Free Speech in it's constitution, but it does have laws against "Subversion of the State" which is a very broad law that basically criminalizes criticism of the state. According to wikipedia, anyone found guilty of this crime shall be sentenced to jail for no more than Five years (< 5). Anyone found to be a ringleader or organizer of such activity shall be sentenced to jail for no less than five years (> 5).