Based on my experience, it is probably fine to 3 months if you keep it refrigerated. There might be some loss of potency at the end.
Titration for tirz is usually to start at 2.5mg/week and increase by 2.5mg every month to a max dose of 15mg/week.
I am in a pickle bc I have a kit of 60mg vials but only take 3mg/week.
As always, your own sensibilities, experience, and comfort level drives what you are willing to research. If something looks funny or makes you feel funny, stop using it.