How soon does the plot of land letter arrive for morthal thane?
How much time does the courier take to give a letter to you after you've become Thane of Morthal?
You don't need to wait for the letter in order to buy the plot of land. The courier sometimes gives it to you at a random time. But you don't need the letter to buy the plot of land. First become a Thane by completing the Jarl's quests and helping people. Then, you talk to the housecarl for the plot of land. You don't need the letter about the Land.