I've been revamping my resume constantly and wanted to get everyone's thoughts on the proper length of a resume.

Taking into consideration the number of years you have been in the workforce (for me 10) and the number of companies (4, but 2 were internet start-ups that failed),  my question to everyone, is how long is too long and how short is too short?

I'm personally at 3 1/2 pages because of the complexity of what I've done, but I know others will be different.

Is there a rough estimate out there by say job type and skills or do HR people tend to lose interest after 2 pages

Thanks