To me and I think *differenty* than most, the replacement schedule is simple.....
Q1: Is the asset able to perform the task for which it is needed?
......Y = continue
......N = sell/trade
Q2: What is the monthly cost of a replacement?
A = $/mo
Q3: Can the asset be repaired for less than the annual replacement cost ($/month * 12 months)?
......Y = Keep
......N = sell/trade
Q4: Will the new asset cost more or less per month, to operate and maintain, compared to the current asset?
......More = think HARD
......Less = buy
If I can keep an existing asset performing well for LESS than the cost of payments on a replacement asset, I keep it. Generally this means that I keep it until it is total junk and no longer suitable for our family (due to size/capacity etc). VERY rarely can you purchase for less than repairs.
I think of it this way: Payments on a new "whatever" are $450/mo, do we spend that much ore more in operating costs + repairs on the current "whatever"? No, then we keep it!
If "whatever" suffers a major failure and can be repaired for less than 12 times the monthly payments on a "new" "whatever" (and in some cases anything close to 12x) then we fix and keep. If the failure truely is catastrophic, then we replace.
****Note: I should add that we don't borrow $$$, we will gladly accept 0% however. The "payments" are the amount per month we would have to save based on the useful life and cost (ie: $83/mo for a $25k asset that has a useful life of 25 years).