Hi!

Please could you tell me how long would the 'cooling fan' motor last
(roughly speaking) if the pc was left running all the time ?

It should last a long, long time. I've got a number of IBM PS/2 computers
that run around the clock. The newest of these was probably manufactured
around 1994. Others range from the late 80s to 92 or 93.

I've never had a fan quit in any of them...then again, IBM used decent
quality fans in their equipment.

The reality with many other computers is that their fans usually don't last
as long. I've seen some quit as early as two years under 24/7 operation. So,
if you do run a computer around the clock, keep an eye on the fans. Pay
attention to any funny noises if the machine starts making them. With
today's computers, loss of a fan can lead to serious damage in a short
period of time.

William