Tosfos in Menachos 35b brings a machlokes whether one must lay tefillin daily or not.
Pri Megadim is mistapek whether the chiyuv is constant or daily – http://hebrewbooks.org/pdfpager.aspx?req=41247&st=&pgnum=61
Rabbeinu Yonah in Iggeres Teshuva Ois 24 writes that one who doesn’t wear tefillin has close to 300 עונשים yearly (excluding Shabbos and Yom Tov etc.)
Smag (Asei #3) and Shibolei HaLeket (Inyan Tefillin) are clear that the Gemara (Menachos 44) which says that “he who doesn’t fulfill the obligation of tefillin is oiver 8 miztvos” refers to daily. (He proceeds to say some very interesting things about tefillin being beikar for reshaim, not tzaddikim… See also Maharik 174).
Ritv”a Shabbos 49 – Chiyuv is Min HaTorah B’shas Krias Shema (or, at least, Kabbalas Oil Malchus Shamayim)
The Brisker Rav – quoted by Reb Moshe Shternbuch תשובות והנהגות ח”א סי’ מ”ט, ח”ב סי’ ל, ח”ג סי” י”א – held that the chiyuv is all day, and there is nothing different about today than tomorrow, so not wearing it for a whole day does not constitute more of a bittul than not wearing it for an hour. See also here