Thanks for all the replies, very helpful.
Am "loaning" out my processor when I am not using it to some research
people, the CPU runs a max for hours.
In terms of wear issues:
1) Pull the battery, before leaving it for long sessions. It probably
doesn't make any difference, but if running off AC, the battery
can be left out. (On my laptop it can - check your manual to be sure.)
2) The fan or fans will run a slightly higher percentage of the time.
3) The "loaning out" thing is more practical on a desktop, because
you can use a more powerful processor (multiple cores) as a
contribution. A desktop is less likely to operate at a high
temperature (at least, a properly designed one).
Some poster left a laptop unattended for a couple weeks, running
at 100%. And one corner of the laptop got so hot, something
melted. So at least at first, keep an eye on things, in case
the laptop really wasn't designed properly.
The CPU has THERMTRIP, so if it got too hot, there is
actually a hardware shutdown. And without the battery present,
there is one less item to contribute to a fire.
If it isn't clear yet, what I'm getting at, it's more practical
to spread the project out into two computers. Use your laptop
for ordinary work (email). Use a desktop to "loan out" cycles, because
it's really designed to run 100% for long periods (up to three
years, until the fans need to be replaced - three years of
continuous operation, is a ballpark figure for fan life).
Paul