Replacement policy based on reliability

Hello,

I am not sure if I'm posting to the right ng, but probably somebody can point me to a good place. I just get assigned a new job as Network Engineer for a company with about 1500 PCs, internet access for every PC, dozens of routers, bridges, switches, CSU/DSU and family.

My boss is asking me to get an exact topology of the whole company with all possible info I can grab about this equipment.

The reason I am posting is that she wants me to get the estimated lifetime of every equipment we have, so I can build the annual budget of the IT department knowing which equipments are close to reach the expected lifetime.

Where can I learn about this ? How people plan for this kind of things ? What are the scientific methods that can back my decision of let's say buy replacement for the 2-5% of the current replacement ?

My personal experience is that a Cisco 25xx for example, works

Well, you should have an idea of what my concerns are by now.

Thanks a lot for any pointers,

REgards Tummy