Depends on how much you
really have hyped yourself on the task :-P.
Back in early 2000s a friend of mine was mentioning on IRC how middle clicking on a scrollbar in Mozilla under X11 didn't jump to the clicked point like in other X11 GUI programs. I was full into the Mozilla hype back then (the project was basically at its apex of coolness :-P) and wanted to get people into it, so i thought "this is opensource, right? I can do it myself and convince people how great Mozilla is".
Problem being, i had a Pentium MMX @ 200MHz with 128MB of RAM. It took at least six hours to do a build (most likely more, i felt asleep at some point during the night, then woke up ~5-6h later, was still compiling and i left to go out with a friend and was done when i came back some time later). Even if i didn't make a change and tried to recompile it took half an hour.
Fortunately i had already done some GUI programs by that point (and even tried to make my own GUI systems and toolkits) so i had a rough idea where to look and it didn't took too long to figure out the CPP file where scrollbar behavior was implemented (though it did take some extra hours).
I did submit it and remember being impressed by the process of having my patch to be "reviewed" and then "super reviewed", thinking that it now makes sense how Mozilla is of high quality (remember, i was a teenager big into the hype, everything was filtered through the most positive lens). FWIW i was asked to make a couple of minor changes but it was merged in.
I don't think i'd have as much patience these days, the last time i tried to build some relatively complex piece of software so i can contribute to it was with Krita, but that took only about an hour (including getting everything needed to build) - and it didn't work at the end (it built but crashed at startup) so i decided to try some other time as i had lost interest by that point to debug why it didn't work :-P.