Shulchon Aruch rules that if one takes off his tefillin, even temporarily, he must make a new brocha when he puts them back on. Rema maintains that a second brocha is not necessary. All opinions agree that if at the time of the first brocha one knew that he would be taking them off and putting them back on, one brocha would suffice for both placements. If the tefillin are removed for an extended period, all opinions agree that a new brocha is necessary.
[סעיף יב, ס"ק מג ו־מז, וביה"ל ד"ה אם וד"ה ויש; ביאורים ומוספים דרשו, 56]