If you take care of your Tefillin and don't lose them, will they last a lifetime?
There are a few different parts of the tefillin, and each has its own lifespan.
There are the "parshiyos" inside, the parchment that has the biblical sections discussing the mitzvah of tefillin written on them.
They have an extremely long lifespan, being that if parts of the writing fade etc., they can be touched up. I would imagine the only irreparable thing is if there is an issue with the parchment.
Either way, these can last 100s of years IINM.
Then there are the "battim", the black boxes that encase the parchments.
These are also made out of hide, but they are more susceptible to becoming irreparable, being that they are required to be a perfect square, and their makeup can lend itself to become misshapen without the best conditions.
Still, these can last many 10s of years, perhaps even close to 100 years if they are taken care of perfectly.
Then there are the "retzu'os", the long leather straps.
These are required to maintain a certain width, and with time and regular use, they become thinner. The rate at which they deteriorate depends on their quality and how they are treated.
I know many people that had to replace them within 10 years or even less.
However high quality ones can last 15+ years. I personally own such.
