I went on HRT for terrible hot flashes/night sweats about 6 years ago at the age of 48, right about the time the big WHO study came out. I didn't care--I was so miserable I felt like I would rather not go on living than continue that way.
Anyway, I'm still on Premphase. I tried a few times to go off HRT but I was just as miserable as I was before starting it. I don't think this is at all related to being on the hormones themselves, though, in answer to your question--I don't believe going on hormones makes hot flashes worse later, and doesn't delay the duration. Your ovaries' production of estrogen is going to "poop out" whether you're taking oral hormones or not.
Lately, I've oddly been having significant hot flashes and night sweats even while on the hormones (however, I'm thinking this could possibly be thyroid related, as I was diagnosed with hypothyroidism and am now on replacement therapy). Some women can go on HRT for only a short time and come off and be flash-free. Often you can look to the trend of your immediate family for what to expect, but not always (as in, what did your mother, grandmother, sisters go through?). At the same time, I've talked to a number of patients (I'm a RN) who were well into their 70's who were still experiencing hot flashes and night sweats off of hormones.
I hope this helps and hasn't just confused you more.