TL;DR For runs longer than an hour, drink lots of water and have sodium in some form. For runs longer than 2.5 hours, add carbs. It qualifies everything with everyone sweats different amount and a different amount of salt.
I have a Gu every 30-45 minutes and tend to drink 0.3-0.5 liters in the same time. The Gu has sodium in it. Hopefully it's enough.