Toward the end of this week's parsha the Torah states, "On the pure menorah he should arrange lamps before Hashem 'tamid' (continually)" (Vayikra 24, 4). Rashi explains that "tamid" does not mean that the menorah must be lit at all times. Rather, it means that Aharon must light the menorah every single night. The Ramban adds that we are to deduce from the word "tamid" which connotes something constant, that the menorah is to be lit even on Shabbos and even when in a state of impurity.
Rav Wolbe (Shiurei Chumash) comments that we can glean from this pasuk the appropriate definition of a "masmid." A masmid is not necessarily one who learns Torah day and night. Rather, it is he who sets definite times for Torah study. It is not the quantity of the Torah study, but the consistency of the Torah study. It is he who sets a concrete time for his learning: day in and day out - even on Shabbos and even when things get tough - who can rightfully be termed a masmid.
Nevertheless, says Rav Wolbe (Alei Shur vol. II pg. 322), there are situations that arise where even one who has set concrete times for his Torah, tefillah, or avodas Hashem, must rearrange his schedule to fit the occasion. For example, Chazal tell us that one must interrupt his study of Torah to fulfill a mitzvah that cannot be performed by anyone else. This rule applies even when the mitzvah arises during the specific time that one has allotted for learning. Nevertheless, one should take a minute or so to prepare himself in a way that will allow him to attend to this interruption in the most time-efficient manner possible, thereby enabling him to return to his schedule of avodas Hashem.
If we have a set time for work, and a set time for meals, all the more so we should have concrete times for avodas Hashem. Moreover, if we are hesitant to let anything get in the way of those times that we set for work and meals, then certainly we should not let anything get in the way of the times we have set (or will set) for avodas Hashem.