I'm brewing beer tonight (in roughly three hours) and I've decided to try rehydrating my dry yeast instead of throwing it directly into the cooled wort. If I boil a couple cups of water, how long does it usually take to get down to 90 degrees so I can pitch the yeast into it? How long can I keep the yeast in the water? How/when do you guys rehydrate your dry yeast?
It really depends upon the type of container you're using, how much water, geometry, material etc. In a glass jar, the water could stay warm for a couple of hours. In a stainless steel jug (I use) it takes about 30 minutes.
What you can do is instead is boil some water now and put that in a covered sanitary container. The water should cool to below 90F before you're done brewing in 3 hours. At pitching time, use a thermometer to check the temperature of the water, and add more boiled water to bring it back up to 90F.
Rehydrating yeast is beneficial to your yeast. You can lose up to 40% of the cells by not rehydrating.