Answer: You should be able to plant them now. However, the tomato you took the seeds from is most likely a hybrid which means that the resulting plants may or may not resemble the parent and the tomatoes may or may not be similar. In addition, there is also a chance that the seed will not be viable, either due to it being a sterile hybrid or due to immaturity at the time the tomato was picked.
Although it is always fun to experiment, if you really want grape tomatoes this summer I would suggest you also try some of the retail seed. Burpee offers Christmas Grapes and the slightly larger but oblong shaped Juliet.
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