Ok, I was looking for this information because someone asked a question similar to it. The basic question is this: How many harvested items can you expect from each type of plant from the moment of the first harvest until it becomes barren? Well, I've decided to test it out for myself. I've just finished the preliminary prep work on it. I've grown every plant except for the death flower and omni all the way to perfect. Now I'm planting 1 perfect plant each and keeping a running total until they all die. I'm keeping them all fertilized with perfect garlic (over 350 of them for the test). I'm going to run this test 3 times with each plant to see if it's a set number or if there's a slight difference between batches. I'll keep updating as I go. Let's hope I don't die of boredom before I'm finished.
Edit 1: Interesting. So far, all the plants that have died have given me fruits in multiples of 5. Lots of 15s and a 20. It's making me think that each plant has an exact set maximum amount, and it'll be the same in all 3 trials. Time will tell.
Edit 2: Okay, I'm finally finished with round 1. I was spot-on with my guess about the multiples of 5. The only one that didn't end in a multiple of 5 is the life-fruit which gave me 12. Here's the breakdown of round 1, which I think will be identical in all 3 rounds:
Watermelon 10
Life-Fruit 12
Lettuce 15
Grapes 15
Bell Pepper 15
Potato 15
Cheese 15
Egg 15
Burger Patty 15
Steak 15
Tomato 20
Flame Fruit 20
Onion 25
Garlic 30
Renoit Grapes 30
Cherimola Grapes 30
Gralladina Grapes 30
Cranerlet Grapes 30
Avornalino Grapes 30
Meloire Grapes 30
Apple 40
Lime 40
Pomegranate 50
Plum (the last one to die by 4 days) 50
Cherry 50
Pomelo 50
Edit 3: Round 2 was identical to the first one. I assume there's no need to run the third round, but since I've taken up the gardening profession, it'll help me get to level 10. Should have the last edit sometime tomorrow.
Edit 4: I've decided I'm going to run this one more time. This time I'll use a less than perfect seed for each and I won't fertilize them at all. Round 3 is almost done, but I need sleep for now.
Edit 5: Okay, Round 3 went as expected, so those numbers are either the maximum amount per plant, or they're the numbers you can expect no matter what. Starting the extra round now with no fertilizer and sub-perfect seeds to see if the numbers change. By the way, just for the sake of full disclosure, my sim has the green thumb trait and the super green thumb lifetime reward.
Edit 6: Wow. As expected, using no fertilizer is making this round take much longer, but I'm actually coming up with the exact same numbers. By the way, off-topic a bit but I was trying to find ways to kill time in between harvests and bought a trampoline. Jumping on the trampoline is a hidden skill. After a couple days, I was doing all kinds of crazy flips that I couldn't do when I first got it.
Final Edit: Being the stickler for detail that I am, I've run one final test. All the previous tests were done by a character at level 10 gardening and with the green thumb trait and super green thumb lifetime reward. Well, I did this final test on a character with level 3 gardening and without either of the green thumbs. It wasn't the whole gamut of items since you need to be of higher level skill to plant them all, but it was enough of a sampling for me. I planted grapes, apples, tomatoes, and lettuce. Each of them gave me the same numbers as all the other test rounds. So my conclusion is this: No matter what the skill of the gardener, the quality of the seeds, or the quality of the fertilizer are, you will always end up with the number of harvested items I have listed above.