In the small truss bridge industry it is typical that the manufacturer will give a 35 year warranty on Hot-dipped Galavanized steel.

You can get more information from the following sites
http://www.usbridge.com
http://www.ovlr.org/jberg/galvan.pdf
http://www.galvanizeit.org

The galvanizeit.com site recommended by rich2001 is an excellent source.  They have graphs of life expectancy for various environments with different thicknesses of galvanizing.  Some project a life of over 100 years.  I have seen galvanized towers from 40 to 70 years old, still in good shape.

The self-weathering steels like A588 may work fine for your application, but it is not a durable surface like galvanizing.  The patina it forms is soft and can be brushed off.  The steel then forms more patina, which reduces thickness.  The patina bleeds, making the surrounding concrete look rust colored.  I don't they recommend A588 in areas with constant moisture without coating the steel (ie direct embedded steel poles).  It is not recommended for lattice type structures due to fraying at the joints.  I personally don't like it, but there may be applications where it is a good choice.  I can't comment on life expectancy, but do not think that it will last as long as galvanized.  

In the UK hot dipped galvanised crash barriers at the sides of roads and subject to de-icing salts last well in excess of 30 years generally. It is the ends of the sections and bolt holes (particularly at expansion joints) which show signs of rusting first.