I got about 5 days on my first run with the powerizers (around 3ml's per day) since then I have only gotten 3 days and that is pretty much consistent. If I am spending a long day away from home, I usually change them before I leave as they last so long I can not always remember the last time I changed them, so it is better to be safe then sorry.
I do not know how long that first set are lasting now as I only used them once and gave them to my mom!