This was my science fair project that I haven't done yet and all I want to know is how long can a plant live in a vacuum
Depends on the plant. For most, not long at all. Seconds, maybe. For others, possibly indefinitely.
The problem with a vacuum is that liquids, like the water in the plant cells, do weird things when subjected to one. The pressure differential means that pretty much any substance with a vapor pressure at the ambient temperature (I'm assuming room temp or similar for your plant) is going to readily evaporate. But, evaporation takes energy; the "Latent Heat of Vaporization"; simply put, most state changes - from solid to liquid and liquid to gas - take much more energy than the "specific heat capacity" of that material in its previous state.
The specific heat of liquid water is 1 cal/g/°C, which is because water is used to define the calorie, as the amount of thermal energy required to heat one gram of water by one degree Celsius (convenient, no?). But, the LHV of water - the energy required to get water at 100°C to evaporate - is 540 cal/g. This is why water doesn't flash into steam all at once when it hits boiling; every gram of water that evaporates into steam is cooling over half a liter of the remaining liquid water by a degree, preventing that water from evaporating until more energy is added to bring that water back up to boiling.
But, if no energy is being added, which is the case in our vacuum chamber, then each gram of water that changes to steam is cooling the remaining water by 540 calories, so depending on how much water is there, one gram of steam can cool the remaining water by several degrees as it evaporates, and there's only so much energy in liquid water to start with. Room temperature is only about 27°C, so a gram of evaporated water can cool up to 20g of remaining liquid from room temp to 0°C.
Now, there's a latent heat capacity required to go between solid and liquid too, called the latent heat of fusion. For water, it's about 80 cal/g. So, the water is cooled quickly, but more has to evaporate to actually freeze it. Starting with 21g, the first gram cools the remaining 20g of water to the point of freezing, but after that, each gram that evaporates only freezes 6.75g of liquid left behind, so you need to evaporate about another 3g of water, then you're left with 17g of ice.
So yeah, that's the weirdness; in a vacuum, water boils and freezes at the same time. Neither is a good thing for a plant to be suddenly subjected to; the combination of water wanting to boil and expand into vapor (water's expansion ratio as it boils is about 1600:1) will rupture surface cells, then the cooling of water in deeper cells that aren't as directly exposed to the vacuum will freeze the water in those cells, and as water freezes, it expands, rupturing the remaining cells.
Some plants, like perennials and evergreens that have adapted for cold winters, have structures in their stalks or trunks that are designed to let the water in the stem freeze solid without harming the tree. These types of plants would likely do best in a vacuum; that's still more than the plant has adapted to handle (there aren't many vacuum environments on Earth even in winter), but they'd at least weather the worst of the damage to the core of the plant, theoretically allowing the plant to recover once a CO2-rich pressurized atmosphere is recreated around the plant.
Most annuals, including most food crops, are not adapted to survive a freeze as a grown plant; they survive as a species by going to seed, and the seed, with very little water, can survive the freeze and then sprout when the soil thaws. The parent plant, however, doesn't survive the winter. Such plants are also not going to survive being freeze-dried; their stalks are less woody, their leaves are larger, and generally they are less prepared for a sudden freeze, so if and when one happens the plant's cells will be shredded by the ice and steam, and the plant won't recover from such widespread damage.
There a lot of factors. A similar experiment can be seen here: https://www.youtube.com/watch?v=H3oyYEp7eTo where a timelapse shows a plant wilting over the course of 6 hours, but as the comments point out, this was a pretty flawed experiment in that it did not tell anyone if the plant was killed or if it could recover, or what the actual pressure was in the vacuum chamber, or even what kind of plant it was since all of these things could change the outcome.
In your experiment, you should consider recording and possible even experimenting with different levels of vacuum and different species of plants. While a plant can not photosynthesis in a vacuum, that would take several days to actually kill a plant. In all likelihood, what we are seeing is the plant losing water to the accelerated evaporation that happens in low pressure environments through a process called vacuum drying.
Commercially, vacuum drying is sometimes used as a method to preserve food. Depending on what you are vacuum drying, and the method involved, it takes ~4hr to reduce a food's water content by 95%, which I assume would be fatal to most plants.