Near the end of the movie, when Hermione lifts the time turner out of her shirt to show to to Harry you can see the chain is pretty short. But then she slips it over Harry's neck and it lengthens, maybe it gets longer or shorter to adjust tot he wearer, but that certain detail isnt mentioned in the books. If it were that long when she had been wearing it previously, the chain would reach somewhere between her waist and knees!