Depends on the tire size. My road bike 25c tires drop about 20-25 PSI per week. I inflate the rear to 105, and a week later it will be around 80. That's normal for tires that size. Other tires like 26x1.5 I've had slow leaks of 20-30 PSI drop after a week or two, which is not normal.

For me it seems like I have to do the water test quite often as I just can't seem to find the leak with my eyes or even ears. I patched a 25c tube the other day, but apparently didn't get it patched well enough. It was losing about 40-50 PSI per day. I did the water test and initially found nothing. Then I dunked it again at the new patch, and found one tiny bubble escaping about every second, to give an idea how slow the leak was. Normal punctures will show many bubbles escaping all at once. So I had to redo that patch, and all is good now.