I think its completely dependent on the brand TBH.
All the "bike" bits are standard MTB bits found on millions of bikes worldwide and even if you couldn't get exact replacements there are always substitutes.
Frames of e-bikes would last for as long as the equivalent push bikes - maybe there is slightly higher wear due to larger speeds or weights
That said even if you can't get the exact electronic components I also suspect that for hub motored bikes there is a certain amount of interchangeability of components. The only real unique thing about most of them is the battery casings.
Most of the kit comes from about 5 chinese companies anyway for every brand of e-bike. And other than the battery many e-bikes aren't rocket science but use electronics technology from the 1980s/1990s..
Perhaps in the medium term LBS's will actually start repairing/modifying e-bikes as well?
Wisper certainly have said they are there for the long term - powabyke is 10 years old and Powacycle are part of a well respected British electronics company (one of the few remaining ones!) which has been around for decades. I would expect all of these companies and their staff will do their best to keep their machines running for many years.
I think some of the German/Japanese crank drive makes also seem durable but I am actually more wary about these bigger corporates as they can walk away from a market without losing too much if they don't think they are making enough profit.
There are Powabykes still running after the best part of a decade, and there's still the odd 6 and 7 year old Panasonic Lafree bikes in use. Before those, the early Lafree E series bikes are still in use at 9 years old in a few cases.
It's the lack of spares or suitable batteries in most cases that's the limitation, rather than anything else that limits their age.
I guess what I was looking for was really was how often should I budget to replace my bike if I use it every day for work and want it to stay reliable?
I'm happy to change the normal consumables like brake pads, chain, tyres, gear/brake cables etc but probably wouldn't want to do too much else.
Does there come a time when things like the electrical wiring and connection just get so unreliable it's more hassle than it's worth to keep it going?
I'm actually hoping to keep my bike updated on a fairly regular basis (say once every 2 years) especially if my company start to offer the cycle to work scheme as they've been (semi)promising. Also I like buying new things
But with some bikes now starting to push towards the £2000 mark I'm thinking it would be worth trying to keep them running longer.
To put my experience into this, my powabyke is 8 years old and in the time owned I have replaced:
3 sets of batteries (about to buy my 4th),
I have serviced the motor (regrease and clean) ,
replaced one controller,
soldered the loom to the controller board due to arcing,
replaced the forks (sheared inside head tube)
new seat post (snapped),
replaced the brakes front and back (poor quality / spring casting failed)
plus tyres, chain, cassette and brake pads as you would expect.
To put this in perspective the first couple of years the bike had an easy life and the battery was neglected (my fault). For the last 5 years it has been used dailyish to commute to work (20 miles round trip) 1500 miles , then 2000 miles , 2400 miles , 2900 last year and currently I have covered 2200 so far.
I have learnt a lot and have had good support (mainly) from powabyke and later this site. I feel the above is fair expectation of the original quality of the bike components used, i am leaning towards a "ping battery" next which will probably see the bike to retirement! (hopefully in a couple of years at least).
To add to Daryl's experience, I owned a Lafree Twist for four and half years, sold it to a forum membrer who used it for a year and then sold it on again so it went into six years of usage. All I did during my ownership was brake pads and recelling one battery at four years.
My Quando based Q-bike has been on the road for nearly three and a half years with no troubles apart from brake pad and battery replacements. Between them these bikes have done over 12,000 miles for me.
For most of us the bike will probably last until we convince ourselves that we really need one of the latest bikes on the market. My four year old electric bike would probably trundle along for years yet, but come the time for the next battery replacement I may well be seduced by a newer, sexier, model instead!