How long will data last when stored on unpowered ide hard drives?

Well, assuming the enviroment is fairly controlled, the hard drive is
not powered, and they still have technology to read IDE in the year
that it's needed... I'd say forever. Or at least long enough.

The harware itself will stop working as well in an average time of 7
years, but that's with constant use, so don't worry about that.

The actual data will last for probably hundreds of years. There is
really not a provable answer to your question. I doubt they've had the
time to run time endurance tests on IDE hard drives in only the
comparatively short time that they've even been around.    SO don't
worry.... Data doesn't just fade away in a sealed and secure hard
drive case over time...

It depends on the environment, drive mechanism and the drive
electronics.
The environment factors are:
Heat-Excessive heat Causes the magnetic media to lose its
magnetism/data
Strong Magnetic fields - Stray magnetic fields from any
magnetic/electromagnetic
device could be harmful.  Earths magnetic field even though is low
could cause
problem on long run.  A magnetically shielded enclosure could be
helpful.
Excess humidity/dust - humidity could enter through the pinhole and
condesation could
occur, leading to oxide formation in the aluminium platters. 
Microscopic dust could be
a problem as well. Air tight enclosure(preferably vacuumed) with a
dehumidifying silica
gel could be helpful.
Vibration - Excessive vibration could cause physical damage apart from
surface scratches.
                 even if the head is parked on a safe region.
Drive Mechanism :
Drying up of lubrication of the bearings, other moving parts could be
a problem.  Rusting/Corrosion
oxidation could also cause malfunction of drive mechanics.  The
magnetic material adhesion
on the platter could reduce in strength.

Drive Electronics:
Except for the electrolytic capacitors the surface mount components
(resistors, inductors and
silicon chips) should be fairly stable.  But if the bios is stored in
a nvram, it could be affected
by strong electic field/radiation.  Depending on the quality of the
manufacturing process/materials
used in the fabrication, the components might degrade in performance
due to contamination/
chemical reactions due to package rupture and atmospheric exposure.
Also the mechanical
stress on the board connectors and components could lead to open
circuits/faulty operation.