So why should I get my windows cleaned regularly?

The most important reason for glass maintenance is to avoid glass degradation.  Glass will, over time, become permeated with a variety of contaminants:

1. hard minerals (from sprinkler systems and building run-off)
2. oxidization (from windows encased in metal frames and screens)
3. acid rain
4. sea spray
5. over spray (from paint, chalking, mortar and more)

In most cases item #1, hard mineral or hard water is the biggest threat to most home and business owners.  The need for basic glass restoration can be avoided with regular cleaning of exterior window glass surfaces.

Understanding glass:

• Glass is porous.  A cross-section of a sheet of glass reveals peaks and valleys.
• Glass is a liquid that continues to harden over time.

Because glass is porous, it's easy to understand how its transparent look can quickly become obstructed with airborne pollutants.

Also, it's easy to understand why hard water (rain water runs over prefabricated surface, i.e... wood stain, painted siding, concrete sealant and deposits minerals and acids from those substances into pores of glass as rain water flows down window glass surface) can damage glass if not maintained on a regular basis.

If you're a home or business owner, I'm sure you can appreciate the necessity of protecting your glass investment.  Glass left un-maintained over long periods of time may result in permanent damage.

So one cleaning a year will help, and two cleanings a year is even better!

-Source: http://www.freewindowcleaningtips.com/why-clean-windows.htm