https://github.com/BOINC/boinc/blob/master/html/user/server_status.php#L441:

$s->users_with_recent_credit = BoincUser::count("expavg_credit>1");

So: RAC greater than 1. Time doesn't come into it.

How long does it take for RAC to drop to zero? Take an average user with reasonable credit in a project, if they stopped today, when would their RAC be 0?

Not looked at how quickly it happens. I think you need to define, "reasonable credit" to get a meaningful answer but my main computer shows on CPDN as being a different machine when I run tasks under Wine and the last ones I completed were in June last year and with 29,697 total credit, RAC is currently zero.

You could set a project to no new tasks and then look at how quickly RAC drops when you do a project update to get a better idea?

How long does it take for RAC to drop to zero? Take an average user with reasonable credit in a project, if they stopped today, when would their RAC be 0?

Never.

It's like radioactivity - it has a half-life (7 days, in this case). So it gets smaller and smaller and smaller, but never vanishes entirely. For the 'recent' question, you mean 'how long until it reaches 1'? It entirely depends where you start from.

Dave's experience doesn't tally with what you said.

This is the BOINC message board. I gave you the generic BOINC answer. If your question was specific to the CPDN project (Dave's chosen specialism), ask at CPDN. If any other project is of more interest to you - ask there.

In BOINC, recent average credit (RAC) is computed as an exponentially weighted average with a half-life of one week. In other words, if an entity is not granted any credit for one week, its RAC will decline by 50%

https://boinc.berkeley.edu/boinc_papers/credit/text.php#:~:text=In%20BOINC%2C%20recent%20average%20credit,updated%20whenever%20credit%20is%20granted.

... and with 29,697 total credit, RAC is currently zero.

No, it shows as zero, because of the forum software only showing 2 digits after the decimal point. It isn't zero.
29,697-> 14,848.5 -> 7,424.25 ->3,712.125 -> 1,856,0625 -> 928.03125 -> 464.015625 -> 232.0078125 -> 116.00390625 -> 58.001953125 -> 29.0009765625 -> 14.50048828125 -> 7.250244140625 -> 3.6251220703125 -> 1.81256103515625 -> 0.906280517578125 -> 0.4531402587890625 -> 0.22657012939453125 -> 0.113285064697265625 -> 0.0566425323486328125 -> 0.02832126617431640625 -> 0.014160633087158203125 -> 0.0070803165435791015625 -> 0.00354015827178955078125 -> 0.001770079135894775390625 -> 0.0008850395679473876953125

Etcetera.

Etcetera.

As soon as I saw Richard's post about it being a half life of seven days I realised it wasn't actually zero.

It may be that only the person who created that statistic years ago would know what was intended.

It has clearly been explained how this number is calculated.
Hence, it's definitely not "meaningless".
If you don't understand it or if you don't like it then simply ignore it.

So the question remains, why is there a "users with recent credit" as standard on all project pages? It's a meaningless number.

It shows who is currently crunching the most for any given project. If you want something else from the server code feel free to ask the devs but if you do so, be absolutely clear about what you are asking for.

It's a statistic. It'll give you a quick view of how active this project is, how many people are actually running it.
The RAC > 1 is primarily used on forums to counter spammers, since the believe is that spammers don't add computers to go crunch data to get a RAC above 1 to then post their spam.

From memory the time period for the "users with returned results" is a project configurable, and most projects just leave it at the default of one day.