The Basics: Orchestral Pieces Most Often Encountered and Approximate Durations 

overture:  Opening piece, maybe 10 minutes, sometimes from an actual opera or theatrical, often a stand-alone piece tending toward the dramatic or heroic. 

tone poem: Can be up to 40 minutes, more stream of consciousness, often based on literary sources or depictions of geography.

concerto:  Solo instrument plus orchestra (usually) most war horses are about 30 minutes, innumerable exceptions.

symphony:  In essence, a sonata for orchestra. Lengths vary greatly: average is about 20 minutes for a Haydn or Mozart, 40 minutes for a mid-19^th century thing by Dvorak or Tchaikovsky, 60 to 90 for Mahler and Bruckner. Anything longer than that at the concert hall is rare.