I think it varies a lot but based on what other people do on my course do I would guess half of them have a job and probably do 12 hours per week based on x2 6 hour shifts and earn ~£8.50 per hour. If you want to know what is feasible for yourself if you study in the UK I will tell you it will depend on your work ethic, type of job, natural ability and course difficulty.
I'll share my experience of working while studying because I feel it represents 2 extremes which highlights why it depends.
I started a Physics course in 2014, dropped out in the second year and then started a Maths course at a different university a few years later in 2019, currently in my second year. I won't get into it but 6 years ago I wouldn't have been able to balance work and uni even on a less demanding course because I lacked work ethic and I know students now who lack work ethic and struggle like I did previously. It's not that these students can't they just won't because they have different priorities and there isn't a lot you can say that will change it.
After I dropped out I had to get a job out of necessity because moving back in with my mum wasn't an option. So I worked and lived with my partner for a few years before going back to uni, I wanted to go back sooner but my job and where I lived wasn't right at the time.
When I started my Maths course I was working 24 hours per week, Friday, Saturday and Sunday plus overtime and averaged 30 hours per week in my first year during term time. It sounds crazy but the work was restocking the alcohol section in a supermarket 16:00 till 00:30 and it worked for me because the job wasn't taxing on the mind and the exercise is great for keeping your energy levels up and improving quality of sleep. That meant I could do all my Uni stuff during the day and then go to work on a night.
This second year I started an internship, it's 16 hours per week with no overtime and better pay per hour but I have found it a lot more challenging. I'm a data analyst in the public sector and the work I do is as important as my colleague's so I'm extremely happy to hold the position. But now I work during office hours, I'm paid to think and the work can be challenging when there is always a deadline. Most days I finish work mentally drained making the rest of that day a write off for anything else that isn't fun and makes me more reluctant to do uni work on other days.
There is definitely a noticeable decrease in the quality of my work between this year and the last that isn't down to Covid and the second year material being harder. My work has just been littered with simple mistakes when rearranging algebra which stings when everything else is correct and it's simply down to I feel more tired this year because of my new job.
Obviously the main thing to consider around this topic is that you can still achieve a good degree so I'll mention that I'm doing very well at uni despite the above. My average scores are above 90% for each semester which I'm extremely happy with because I have worked very hard to achieve that and have worked hours my friends wouldn't even consider.
So to summarise you can still get the full uni experience (societies, nights out, etc) if you work ~10 hours per week somewhere casual and have a good work ethic. So if you're thinking about I would definitely encourage you.
I definitely don't recommend that anyone should work the hours I have because it definitely isn't sustainable over the course of a full degree, I only worked the hours I did out of necessity. Since I have 2 years previous study I have to pay a full years tuition before the end of this degree and I also can't move my girlfriend and dog into a student house so between the tuition, council tax, travel, extra rent and utilities my outgoings are very large.