I don't make a habit of it, but during some periods when too many demands
were being made on my available time, I have inadvertently left shells in
bleach for a month or so, with no apparent ill effects.  There are some
exceptions.  Very thin, poorly calcified shells like Hydatina (paper bubble
shells) or Philine, and some land snails do not do well in bleach.  Also,
the interiors of Haliotis (abalones) seem to lose some luster in bleach.
Bleaching of cowries can also be problematic unless special care is taken.
But for most shells, extended exposure to bleach does not seem to cause any
harm, in my experience.
The reason it takes so long to bleach out small, slender, multi-whorled
shells like Terebra (augers), Turritella, and many Epitonium (wentletrap)
species it that when you lower them into the bleach solution, air remains
trapped in the shell, and acts as a barrier between the tissue you are
trying to dissolve, and the bleach.  In larger shells this is not usually a
problem.  Nor is it a problem in smaller shells that are more widely open,
like Natica, Littorina, Cancellaria, etc.  Try holding the shell with a pair
of long forceps (tweezers), and swishing it back and forth through the
bleach solution a few times, before dropping it to the bottom.  This will
usually flush at least some of the entrapped air out of the shell, allowing
more bleach to get in.  Or, flush the shell under a high pressure stream of
water, and do not shake out the water afterwards.  Allow the shell to remain
full of water.  Then place it in the bleach.  The bleach will diffuse in
through the water, replacing it.
I personally use an outstanding method which easily cleans the interior of
narrow shells, right down to the apex.  Unfortunately, it requires some
special equipment that most folks don't have around the house; but I'll
describe it anyway.  I use a vacuum oven, which is essentially a chamber
designed to withstand the force of a full vacuum, and a powerful vacuum pump
to provide the vacuum.  The vacuum oven can also provide heat (hence the
name "oven"), but I don't use heat in cleaning shells - just vacuum.  I
place shells like Terebra (augers), etc. in a glass or plastic beaker, the
bottom of which is filled with glass beads (clean sand would also work) to a
depth of about 2 inches.  I push the apex end of the shells straight down
into the beads, which holds them in an aperture-up position during cleaning.
Then fill the container with bleach solution, and place in the vacuum oven.
Turn on the pump, and evacuate the oven (remove all air).  This causes any
air inside the shells to expand greatly, and be forced out through the
aperture.  Then the bleach can flow in.  Bubbles of gas are also formed by
the action of the bleach on the organic matter inside the shell.  You have
probably noticed this while cleaning shells in bleach.  However, under
vacuum, these bubbles come out of the shell at a greatly accelerated rate,
bringing with them many bits of partially dissolved organic matter.  You can
see this because the vacuum over has a glass viewing port in the door.  When
the bubbles stop, the shell is completely cleaned.  Then you just release
the vacuum, remove the beaker, and rinse out the shells with water as usual.
An average size auger will usually be clean in about 20 minutes, unless the
whole animal was dried inside the shell, in which case it might take an
hour.
This method also works great with microshells, which it is almost impossible
to clean internally any other way.  You don't necessarily need a vacuum
oven.  A simple vacuum jar will do, and is much less expensive.  However, a
good vacuum pump is still fairly expensive.  WARNING!!!  UNDER NO
CIRCUMSTANCES SHOULD YOU ATTACH A VACUUM PUMP TO ANY CONTAINER WHICH IS NOT
SPECIFICALLY DESIGNED FOR THAT PURPOSE!  The container may implode, causing
serious injury to anyone nearby!

Paul M.