Yeast grows quickly in water that feels a little warm. If
the water is colder, the yeast will grow more slowly. If the
water is too hot, it will kill the yeast.
The flat shape of the plastic bag has a lot of area in proportion
to its volume. It will come to room temperature quickly. If you
want the yeast to work more quickly, you may want to wrap it
in a towel and put it on top of a heating pad or hot water bottle.
Be careful not to get the yeast too hot: it will die.
If the room is cool and you have no way to keep the yeast
culture warm, wait until next morning to see the bags puff up!