I planted a Forbidden Fruit seed to try to get a Plantsim baby, and it's the second day of winter in game and the plant is at the mature growth state and has gone dormant. How long before I can harvest the plant?

Plants stay dormant until about half way through the spring season. If you have Supernatural, you might be able grow forbidden fruit indoors in the planter bowls (which would take them out of dormancy the next morning) but I don't have University so I can't check for certain.

You don't need SN to use planters, as long as you are patched then you can use any of the custom planters available out there.

I do have Supernatural and therefore planters, but I don't know how to use a planter. If someone could explain how to use one, that would be great. I'm not new to the sims by any means, just never really gardened before on the game, just did this to try to get a Plantsim baby.

Since you already have the seed planted go in to buy or build mode, select the plant, and move the plant to the bowl. If you were wanting to plant a new seed you would click on the seed to select it for planting, like you would normally do if you were outdoors, and move the seed over the bowl (I almost always forget and try to click on the bowl instead).

Trees can't be put in planters, which is why I'm not sure you could plant a forbidden fruit in one. But if nothing else, you could use the moveobjects cheat.

Build a little house around it with a roof. Then it's immune to winter and will start growing again. You have to break the wall if you want to plant more, tho'.
If you have supernatural, you can plant it indoors in a planter.
If you have into the future, you can plant it on a green carpet patch.

The only thing you can't plant in planters/indoors are trees. (Unless someone has found a workaround?)

Are you sure the forbidden fruit plant can be put in a planter or carpet patch?
Edit: I was able to put the plant in a planter. Thanks everyone for the help!

I know that this is a huge necro, and I apologize, but this is one of very few threads that comes up upon google search so I think that it's best for me to put this solution here rather than making a new post. You'll need to have NRaas Retuner installed.

What you do is go to your sim's computer, open up NRaas Retuner, and then go to Settings > General > By Tunable XML > Sims3.Gameplay.Seasons > SeasonsManager+TreesManager. From there, you'll want to make the following changes:
kSpringLeavesRegrowTime 0
kFallLeavesStartTime .75
kFallLeavesEndTime .99

The number is a percentage in decimal form. 0 is the beginning of the season, and .99 is the very end. Tree dormancy is dependent on whether trees have leaves or not, and the sooner they regrow, the sooner the trees will come out of dormancy. Those settings are just the preferences I chose, so you can tweak it as you like. Changes take effect at 5am when plants update.

Next time Fall comes around I plan on having a long one, and I will test if kLeavesLossAmount changes much. My guess is that this is a range for how much leaf coverage is lost each day, but since I tested originally with a 7-day Fall, it didn't affect anything.

As I expressed on the EA forum where you also posted this (and that one might get locked down since the moderators there hate necro posts no matter how helpful they are intended to be), I don't think that leaf loss by way of the TreesManager really equates to harvestable plants going dormant.

Fancy meeting you here as well, lol. As I stated in the other thread, though in different words, kSpringLeavesRegrowTime works for fruit trees at the least. The tree was no longer saying it was dormant after I changed the setting, even though it still had that appearance, but it didn't fully take effect until the next day, when I was able to see leaves and harvest.

I understand, but you are also saying that these trees continue to bear new fruit (and weeds) that weren't there before and growing trees can continue to pass through their life stages as a result of these settings? Or not enough time has passed to tell that yet? If so that would be surprising, but could be very helpful.

I was using NRaas Debug Enabler before in an attempt to switch growth stages. Prior to changing kSpringLeavesRegrowTime it would switch between mature and harvest, but still remained dormant. So after I changed that, I switched its stage to mature that way it would be ready for harvest the next day. I haven't had issue with it since then, even changing its growth stage. I think it's fixed. I switched the season to Fall after learning how (as it happens, I saw another post of yours on the other forums stating how). I'll let you know how the leaves are affected.

Edit: I went through a 7-day Fall, and things basically went how I originally expected them to, with the exception that the leaves on the fruit trees didn't disappear at all until the first of Winter at 5am. I would have needed to change kFallLeavesStartTime to around .80 or a lower number to make the leaves even start to disappear on a 7-day Fall. And since the leaves remained in tact, the trees didn't go dormant even once.

Upon the beginning of Winter, the leaves disappeared all at once and the trees went dormant. I skipped ahead to Spring and the fruit trees were fully revived at 5am, though regular garden plants like Life Fruit remained dormant until the next day.

Side note: kFallColorChangeEndTime affects how late into Fall the leaves on harvestable trees changes to Fall colors.