The time taken by avast's on-demand depends on several factors, mainly the Sensitivity, Quick, Standard or Thorough, add to that if you select scan archives. Your system CPU and RAM also play a part.
The thorough is also by its design very thorough and perhaps a little overkill for routine use, were a Standard scan without archives should be adequate.
I have only ever done a through scan with archives once shortly after installation just to ensure a clean start state, but with XP for example avast will do a boot-time scan after installation if you select it, this I believe will be quicker and reasonably effective. Like everything in life things are a compromise.