Here is an example of what I was looking for:

Equipment Type       LifeSpan

Monitor                     10 years

Cart                           10 years

Chromebook              5 years

Desktop PC               5 Years

Digital Camera          5 years

Document Camera    5 Years

LCD Projector            7 Years

Printer                        7 years

Ipads

MacBooks

IMacs

Mini Macs 

etc....

This is not a complete list, but I'm just trying to get a general idea on what other districts are using for standards, and if possible, some information regarding how that lifecycle was determined would be helpful. 

For example, Google sets the supportable lifecycle of a Chromebook at 5 years, but perhaps you set it to 3 because student's take those systems home and there is additional wear and tear associated.