The number of years should be between 0 and 5 but no validation takes place

 1 static int GetHowLongToRun()
 2     {
 3       int Years = 0;
 4       Console.WriteLine("Welcome to the Plant Growing Simulation");
 5       Console.WriteLine();
 6       Console.WriteLine("You can step through the simulation a year at a time");
 7       Console.WriteLine("or run the simulation for 0 to 5 years");
 8       Console.WriteLine("How many years do you want the simulation to run?");
 9       Console.Write("Enter a number between 0 and 5, or -1 for stepping mode: ");
10 
11       while(true)
12       {
13           Years = Convert.ToInt32(Console.ReadLine());
14 
15           if (Years >= -1 && Years <= 5)
16               break;           
17           else
18               Console.Write("You MUST Enter a number between 0 and 5, or -1 for stepping mode: ");
19       }
20 
21       return Years;
22     }

Explanation

Line 11: While loop added so that if the user inputs an invalid number, there is a way for them to input again, an unlimited number of times.

Line 15: Checks if the value of years is between -1 and 5 (all valid inputs).

Line 16: If line 15 is true, the program will break out of the while loop and continue as usual.

Line 17/18: If the above if statement is false, the user will be prompted to add a valid number.

Other Method

 1 static int GetHowLongToRun()
 2     {
 3         int Years = -2;
 4         Console.WriteLine("Welcome to the Plant Growing Simulation");
 5         Console.WriteLine();
 6         Console.WriteLine("You can step through the simulation a year at a time");
 7         Console.WriteLine("or run the simulation for 0 to 5 years");
 8         Console.WriteLine("How many years do you want the simulation to run?");
 9         Console.Write("Enter a number between 0 and 5, or -1 for stepping mode: ");
10         
11         while (Years < 0 || Years > 5) {
12           Years = Convert.ToInt32(Console.ReadLine());
13           if (Years < 0 || Years > 5)
14               Console.WriteLine("Try a different number");
15         }
16         return Years;
17     }