Could anyone please tell me whether leaving a computer on standby for a long
period of time, approximately 12 months, would harm it in any way?
In general, no it won't harm it.
HOWEVER, it also depends a bit on the construction of the
system. For example a poor quality power supply,
motherboard, (etc,), or random defect in a part can cause a
failure unnoticed by anyone then as the failure progresses
it could cause further damage than if system had been turned
off and fixed at that point.
Further there are some maintenance items to consider like
failure of fans or dust buildup. Consider what the
environment will be like for that period of time, the
extremes of humidity and temp for example.
It's not hard to prevent these things but due to the nature
of PCs, their widely varying parts, I'd not want to bet on a
cheap box lasting a year before examining it. It might also
depend on what you mean by "standby", as S3 keeps a lot of
power going through the PSU's 5VSB circuit and might be
harder on the system than if it were full-powered all the
time.
Why is it so important to leave it on standby? If it's just
a light webserver you might put together a Via Eden based
platform to replace it, without a monitor and with single
HDD, underclock it and disable power management... much
lower heat allows much lower fan RPM which means longer fan
life and less dust buildup in addition to lower energy
waste.
