You will get 5000 Nethershards per one hour (can last longer, especially if you don't have flying in Legion unlocked).
Any items which reward any number of nethershards on use or upon turning in are counted towards total quantity of Nethershards to be farmed.
For 5000 Nethershards you can buy Relinquished token, which rewards you 880+ ilvl epic gear and also gives you a chance to target legendaries to a specific gear slot.
NETHERSHARD ESSENCE material which is used to craft Legendary item (costs 8000 Nethershards).
You can see a full list of rewards which can be purchased for Nethershards currency NETHERSHARD.
110 Lvl Character. Don't have one yet? Take advantage of our LEGION 100-110 POWERLEVELING service.