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Most mirrors used around big cities gather a greasy film fairly quickly. Just breathe on and look at the condensation pattern to see if it has a greasy film .A patchy blotchy condensation pattern, means the mirror is greasy . Also those in bushy setting get a film of Eucolyptus oil which is very tenacious. The mirror surface needs to be soaked in methylated spirits before the normal cleaning process is started. Greasy films and dust scattter light and lower planetary contrast. Small scattered spots of dirt or previous washing blems don't matter.
On the whole most mirrors I have seen get left way beyond the point that they are in need of a good wash for some fear of the unknown. They are all overcoated these days, so you aren't going damage the glass surface of the mirror. IMHO If you like your images clean and contrasty wash your mirror as often as needed, and don't deem it clean unless it passes the breath test.
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