I've recently spotted a relevant research opportunity in my discipline, and it's all online due to COVID-19. That's great for me as I don't live near the particular university offering the role, and I am self-funded.

However, I am slightly concerned about the number of hours the role will take per week (as I am doing my PhD full-time). I would have to work every day for 3.5 hours, as well as some Saturdays. I'm worried that my own project might suffer, although the position is only for 3 months and therefore wouldn't be forever. I also don't know how happy my supervisor would be about it.

Has anyone been a research assistant during their PhD? How did you find it in terms of workload etc.?

When I was doing my Ph.D. I did about 9 hours of labs and a few hours of marking - often just a weekend day for marking. In my first year and second years about 3-4 of us Ph.D. students would deliver the Java programming labs which were originally a full 5-hour single session with the whole of the undergraduate Computer Science students. I and another student helped deliver the M.Sc Data science labs (large-scale computing stuff, distributed computing, bigdata etc..), designed and marked the coursework. It didn't negatively impact on my own work, but I stopped all teaching during my write-up at which point I recall just marking for just two modules.

I am currently doing a full time research assistant role and have suspended my studies. Fortunately, it is with my PhD supervisor at the same uni on a project that is close enough to my PhD that I can include it in my thesis.

If you are self funded already, request a suspension of studies (or whatever your uni calls it) and keep working on your PhD. 3.5 hours a day is a significant amount of work and will delay you even further but if you can suspend you don't lose any money or PhD time.

I meant I did 9 hours per week.

That is a shame PhoenixFortune that a suspension would not work. Though if the 17.5 hour a week job is going to land during data collection anyway, that is going to be a crazy amount of work simultaneously. If you need the money to enable yourself to finish the PhD, I would talk with your supervisor if there is anything you can do to make it work. You might have to modify your planned timeline slightly but there should be a way to for you do take the opportunity, without burning out.