Hello,
I am an Android Developer with 6+ years of experience. I took a look over the app's link and the UI looks pretty straightforward, however after reading the description I see that there is heavy logic behind it. Since I do not know the nature of the bug, I cannot make any promises to how quick I can fix it. Usually it takes me less than 20 minutes to fix the average bug when I know the source code. I your case it would be about an hour, if it's a standard bug with reproducible steps.
Thank you,
Alexandru