If we measure particular aspects of our disciplines in order to simplify our spiritual lives or to hold ourselves accountable to certain goals, then there may be real benefits. So a person might try to read a given number of chapters in the Bible daily in order to avoid deciding every single day how much to read, and/or to keep pace for reading through the Bible in a year.  Not even the most rigorous practice of the spiritual disciplines is legalistic when the motives of our spirituality are what they should be, namely to do all to the glory of God and to pursue Christlikeness.