validation expression for at least 150 characters

You can use Range in the DataAnnotations attribute.
This will set the range, else use the MaxLength property and set to 150. This should work.
If I have anticipated correct.

[Required(ErrorMessage = "Name is required.")]
[StringLength(40, ErrorMessage = "Name cannot be longer than 40 characters.")]
public string Name { get; set; }

Accept if helps anyway.
Thanks

        .{150,}


dot for any character, 150 minimum, no maximum.