what starts to go wrong, in general, is that lenses begin to accumulate
deposits on them and become uncomfortable.  different materials behave
differently.  different individuals can use the same lens materials for
differing times depending upon the physiology of their eyes.  for
example some people with bad allergies will get mucous deposits on
their lenses within a matter of days while others can wear the same
lenses for more than a month without noticable deposits.

however replacement schedule is also influenced by marketing.
manufacturers would like you to throw away their lenses often and buy
replacements so they like to recommend short replacement schedules.
however sometimes manufacturers choose to charge a higher
price-per-lens for their contacts but claim that they last longer by
giving them a longer replacement schedule so as to defend against the
price objection.

a perfect example is the 1-day Acuvue lens.  the exact same plastic is
used as is used in the 2-week lenses that Vistakon sells (Acuvue and
Acuvue 2 and Surevue).  yet the company claims that these lenses should
be worn only for 1 day and then replaced.  it doesn't take a genius to
realize that the 1-day lenses can be stretched out to 2 weeks (or more)
and you can save a lot of money!  Same with B&L's line of polymacon
lenses which are virtually identical but are repackaged, named
differently, and labeled for 1 year, 3 month, and 2 week replacement
schedules.  some years back this was the subject of an expose on an
episode of 60 Minutes!

although the Biomedics XC and the Proclear materials are not identical,
they are quite similar.  i "imagine" that their replacement schedules
would be identical for your personal physiology.  you need to try them
yourself and see how long they last and how comfortable they are.

and you should also know that wearing old dirty contact lenses can
physiologically affect your eyes much in the same way as extended wear.
they can cause inflammation and corneal edema.   don't go crazy
wearing disposable contact lenses for many months and think you are
doing no harm and just saving money.  in general, I think a 2 week
disposable lenses can usually be used safely for about a month but the
proof is in you personally seeing how your eyes react.

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