Thats a LONG life cycle you have for both ios and os x devices.
Most education/corporates i know of adopt a standard 3 year cycle for Computers (Laptop/Dekstop) and a 2 year cycle for tablet (ipad) devices. This also works with the tax system of writing off computer equipment at 33.3% per year. Also factor in the 3 year warranty for education computers and 2 year warranty for education ipads. It doesn't really make sense to hold on to them for longer.
Most places also like to stagger their purchasing, so that there is not a massive refresh of devices at the 2 year and 3 year anniversary. It would be good to be purchasing new machines every year, but not have a computer onsite that is over 3 years old and not a ipad that is over 2 years. Hope that helps :)

Thanks. That does help. Is there anyone out there that does a two year replacement cycle or possibly a three year cycle for replacement of tablets that could give me insight into how everything goes with this?

I work for a community college. I'm curious about this topic as well.

One area I'm currently supporting hasn't had any rotation system. Here in Illinois budgets haven't looked good for some time and it just got worse. All Macs and now iPads have been in a "use it until it dies" system which is pretty crazy. I inherited one lab that is using 9yr old iMacs for some music instruction running 10.4.11.

On the Windows side which is in other depts I support rotation is all over the place. If they have money, it's 3-5 years...in others maybe 7.

I don't know about other school systems but the way our budgeting is done is by dept and it competes with everything else so some depts see the value in a rotation system whereas others prioritize IT purchases low and deals with it as it comes.

In our corporate environment, PC's are down to almost a throw away item in the last couple of years, prior to that, it wasn't uncommon to have a 8-10yr old pc (a lot of printers were even older).
Now I believe our PC cycle is 3years. Macs were always treated as a niche product and only refreshed when necessary. Since I took over, I got them to a 3yr refresh, but that seemed to be slipping to 4-5 year due to politics. With some senior management that was pushing for the longer refresh no longer a roadblock for me, we shall see what gets cut from my budget this year and hopefully get it back down to 3yrs.

Of course it's hard to make a case when we can buy 3 PC laptops for every 1 of our "standard" Mac laptops.

Our educational institution generally uses a 4 year replacement cycle for computers and 3 year for iPad. We haven't done an iPad replacement cycle yet as we're really only in our second year of mass deployment of those devices. We generally try and replace 1/4 of all our computers each year. So we do a new computer deployment every year. I'm curious if others are replacing all their systems at the same time or if they are also spreading them out over the replacement cycle as we are doing.

mpermann, for us it's every year that our campus is getting new computers and then we rotate what gets replaced to somewhere else on campus if those systems are usable. It keeps the volume down for getting new computers at once and we already don't get a very big window to install them in(1-3 weeks usually).

We have a 5 year replacement cycle for PCs and Macs (K-12). We try not to reuse the old equipment elsewhere, but sometimes it cannot be helped (especially with computerized testing becoming mandatory soon, and the always increasing need for more computers). At my old job (non education) we had 8 year old PCs and they were just too slow, which impacted efficiency even though they were higher end when purchased.
We are only two years into iPads.. but I don't necessarily see us doing a 2 year refresh as standard. For our 1:1 we may sell at 2 years and see if we can get 50% value and buy new..

I'll chirp in from the enterprise world. Our replacement cycle is 3 years expected 4 years max. After three years.. Applecare is over, and users can get a new machine. That being said, we don't offer them the machine, its up to them to check into it and follow up with us, at the best time for them.

TL;DR Generally. Its 3 years +- 2 months.

as for iPads, we're just starting to roll them out.... We haven't really set out a replacement schedule for them.