Extend, lengthen, elongate, prolong, protract all mean to draw out or add to so as to increase in length.
Both extend and lengthen (opposed to shorten ) connote an increase of length either in space or in time, but extend is also used to connote increase in range (as of kinds, of influence, or of applicability); thus, a road may be extended or lengthened; one may extend or lengthen his stay; the power of a monarch, however, may be extended but not lengthened.
Elongate (usually opposed to abbreviate ) denotes to increase in spatial length and has wider technical than general use.
Prolong (opposed to cut short, arrest ) means to extend in duration beyond usual or normal limits.
Protract (opposed to curtail ) adds to the denotation of prolong the connotations of indefiniteness, needlessness, or boredom.