The time required to make a harbor telescope is judged to be 2 weeks (including making the frame that holds the telescopes mirrors). For each additional foot in length and corresponding magnification increase of 10x, double the length of time to manufacture - thus a six-foot long telescope with a magnification of 50x would require two months to fully construct.
(while this formula certainly bears NO relationship whatsoever with the actual manufacture of telescopes, or their magnification, I lack any meaningful, useful information for the making of various power and size telescopes with 17th century materials and techniques. Therefore, for gaming purposes only, the formula is purely practical).
Note that all telescopes made in my world are refracting telescopes - Robert Hook has not yet invented a functioning reflecting telescope.
See Astronomy & Astrology