how long can a string be?

public StringBuilder(int capacity)
{
      this.m_currentThread = StringBuilder.InternalGetCurrentThread();
      this.m_MaxCapacity = 0;
      this.m_StringValue = null;
      if (capacity < 0)
      {
            throw new ArgumentOutOfRangeException("capacity", string.Format(Environment.GetResourceString("ArgumentOutOfRange_MustBePositive"), "capacity"));
      }
      if (capacity == 0)
      {
            capacity = 0x10;
      }
      this.m_StringValue = string.GetStringForStringBuilder(string.Empty, capacity);
      this.m_MaxCapacity = 0x7fffffff;
}