Is managing a open source project more time consuming than a personal one?
Users keep asking me to make my app an open source project (and I have no problems with that), but I am afraid that it would be too much time consuming for me as I maintain 3 apps on the Play Store and also study and practice sport.

Well I suppose you can try and give up if you can't keep up, or have a co-maintainer.
But I don't think it will take THAT much time, you probably won't have hundreds of collaborators (even thought I hope you do) !

Dfox, is that you?

No not really and if you use the right automated testing tools it can be as easy as just pulling up pull requests that pass and merging them but if you do do it this way i suggest there's code quaility, code coverage, lint check, and unit test along with a build check if you want it to be real easy for you. On github all of these have free versions for opensource projects so there's not alot of reason not to besides the inital setup time and even then i believe there's a way to auto pull pull request if all test pass(not recommending) and google play has an update api where you can setup an continous deployment pipeline also. So really an app could be setup to be fully automatic where its set it and forget it but i would suggest if nothing else manually approve pull requests where backdoors don't get put in.