הלכה על ברכות 18:13

The time for putting on tefillin is like 'Others" in Berachot 9. One should wear them all day, and even to do business in them is permitted, and snacking is also permitted. But idle activities are prohibited. If one is worried this seems haughty before other people, one may wear only the hand tefillin. In any case, it is better to wear a small head tefillin and keep the straps covered than to walk with only the hand tefillin.