How long is tretinoin cream effective for? i have it about a yr & it doesn't burn or irritate even if used on new skin like the arm.

Some basics: This product like many similar ones comes in differing strengths. They generally have an effective shelf life of a couple years. Effect on regular thick skin like your arm is much less than the fragile tissues of your face. Leave it on too long & you may burn your facial skin. Use it for the duration you have developed a tolerance to & it may just do its job.