Echoing what Sycorax noted and what has been explained in this Meta post:
How long do suspensions usually last for?
It really depends on the mod or the user, but usually, via the system-recommended suspension lengths, the first will last for 7 days, the second 30 days, and the third 365 days.
From that same post,
A network-wide suspension ... can be longer than 365 days.
[There can be bans for longer (way longer) durations for drastic cases, for instance, a temporary suspension ending in the year of $2292, $ mainly due to the absence of any feature akin to permanent ban.]
Suspension is imposed when all other reasonable options have been exhausted. It's not that the OP is not made aware of the consequences of their deeds.
There can be varied reasons to invite the spectre of suspension but broadly speaking:
[...] an account can be suspended for a protracted amount of time when all other attempts at reasoning have failed when it comes to a certain type of behavior. ...
Also as has been noted in this Overflow blog, these can be primarily due to tue user showing no effort to learn and improve over time and involving disruptive behaviour.
Related post: What kind of suspensions are there on Stack Overflow?
$\bullet$ You claimed:
Should all users be treated the same way, irrespective of their previous contributions? Suspension can feel as a very agressive sanction, in particular as it is publicly visible and the user's rep is shown as 1. This would not make much of a difference for a low rep spammer or a beginner, but for a loyal contributor it must be very frustrating.
Again, I would like to allure your attention to that old blog (emphasis mine):
Our general strategy is to discourage specific problem behaviors, not individual users. But sometimes you just can’t seem to reach people, and it becomes necessary to place accounts in timed suspension. ... We don’t hold grudges. The point of all this is to address the behavior. If the behavior improves, you are welcome back.
I don't view suspension as a means of public shaming; this is a derogatory term that should not be synonymous to suspension. It's one of the last resorts that attempt to provide room to the said user for retrospecting and being pensive over their actions/behaviour.
Enforcement of norms and standards in this community driven site should be the foremost priority if it needs to flourish in a sustainable manner. There is no two tier system for users with high reputation and for those who are newbies - if they persistently break the rules and engage in a way not warranted, proper actions should be taken by the moderators. Recently, for instance, in our sister site, Maths, on account of constant influx of low quality homework/assignment questions and subsequent answering by high rep users without bothering to find any possible duplicate or to flag/vtc, the mods have decided to amp up their involvement to avert this plague. Check their meta post: Enforcement of Quality Standards. This resulted in suspension of some very high rep users. This was not done to vilify them, shame/embarrass them publicly, but rather was a purportedly strong step to deter them from the activities that were considered to be hindrances for making the site a quality repository of good Q&As.
$\bullet$ You asked:
Have attempts been made to introduce some less fierce sanctions, such as limiting the number of allowed posts per week/month tacitly for a while, and allowing mods to modify the sanction for low quality posts on a per case basis? Would this be feasible? What do you think?
Now, my quick search didn't land me to any post that proposed any alternative route advocating less fierce sanctions (I am not saying there are none). If you want to suggest some, I would urge you to ask this in Meta.SE.
Remember as this post notes:
... we once didn't have suspensions, the thinking was that if a user was that much of a problem, they should probably just leave the site. That thinking changed quickly as we learned that very awesome people sometimes just have difficulty controlling their emotions, or folks learn that using sock puppets was a pretty bad idea and just participate honestly.
When used correctly, suspensions should just freeze the action, let us inform the user of what's wrong, how they might fix it and lift automatically after a short time. Using them for anything else is generally a bad idea, and we've moved away from that over the years.
Suspensions aren't imposed hastily. Rather it involves detailed scrutiny of the activities of the said user, their failure to respond to any possible warning etc. So, imo, I don't think there is any necessity to moot about any alternative. Dire situations demand dire actions.
Lastly, throughout the post, I have tried to articulate my opinion as well as the established facts here and tried to explicitly discern the two. However, I think I am in full agreement with the other.