lori, you will hear different things about the longevity of lev because many of us have had different experiences using it. one of my flexpens lasted only 34 days... another lasted 37 days before seeing floaties (less than the manufacturer's suggested 42 day use). however, the flexpen i'm currently using was opened on 09-10-2010... almost 4 months old and still going strong. go figure. i've handled all of them the same.
some have had to toss a flexpen when the little disc inside the pen reaches about the 12 mark. others have been able to use a lev flexpen down to the last drop. my advice is to visually inspect the pen often. toss it when you see floaties or it becomes discolored or if it has frozen. if there's no possible explanation for an unexpected rise in numbers... you might want to try a new pen.
as far as shooting excess levemir back into the flexpen or vial... many do it. i'm probably one of the few who would not shoot
any insulin whether it's levemir, lantus, or pzi back into the pen/vial because of the silicone oil which has been sprayed on the inside of the barrels of disposable syringes:
http://www.henkesasswolf.de/fileadmin/pdf/Latex_and_Silicone_Oil_Report_on_HSW_NORM-JECT_R_.pdf.
human diabetics are known to build up a small amount of silicone oil under their skin from injections. i don't see any reason to possibly push silicone oil back into the pen/vial only to draw it up again the next time i pull an insulin dose. i'm sure the amount of silicone is minute, but why do it if i don't have to? like i said, i'm probably one of the few who is concerned not necessarily about the efficacy of the insulin itself when shooting excess insulin back into the vial/pen, but rather have an increased concern stemming from the injection of something other than insulin into my cat's body. different strokes for different folks.
most lantus and levemirs will tell you it's best to stick to a 12/12 schedule whenever possible. ideally, it's the best way to go. however, life isn't always ideal. contrary to popular belief, there have been
BOTH lantus and levemir users who have had to become (out of necessity) somewhat creative concerning shot times and dosing. it's not easy, but it can be done. lev AND lantus kitties who are tightly controlled are more forgiving when it comes to being off schedule.... so much so that my own tightly controlled cat rarely misses a beat in the event of a skipped shot.
good luck with the switch to lev! i think you'll grow to like it.