Okay, I'll go off ow what I've said above.
So, according to here, after you water a berry, it dries up after 4 hours.
It also says that the minimum yield for a Starf Berry plant is 1, while the maximum yield is 7.
According to this, it takes 48 hours for a Starf Berry to fully grow.
Knowing this, we can find out how many times you have to water it.
48 / 4 = 12
So, you would have to water the Starf Berry plant 12 times for it to be fully watered.
Now comes the more confusing part.
According to this, it says:
>A Berry's yield depends entirely on how many times it was watered. Soil dries out four hours after it is watered, meaning that a Berry's yield is equal to:
yield = (max - min) / (time / 4) * water + min
where max and min are the Berry's maximum and minimum yield, time is the Berry's total growth time in hours, and water is the number of times the plant was watered.
So, we now know what we have to do to find out what the Starf Berry's yield will be if you have watered it all 12 times.
(7 - 1) / (48 / 4) * 12 + 1 = 7
So, if you water the Starf Berry plant all 12 times, you'll get the maximum yield of 7 berries.
I tried replacing 12 (the amount of times watered) with other numbers, but to get maximum yield from a Starf Berry plant, you would have to water it 12 times.
However, if the amount of times water is 11, you get an 6.5 for the amount of berries you the Starf Berry plant will yield. I'm not sure what the game would do with 6.5, but my guess is that it would round 6.5 down to 6.
Hope this helps!
I've listed them above.