At least ten years. By then they'll probably have decided it has to be forever.
Either one is a nasty prospect over ten years. Tamoxifen doesn't work as well and raises my risk of blood clots, cataracts, and retinopathy (uncommon but as I say, I'm the Primrose Everdeen of side effects). Both cause hot flashes and joint pain, although for me the tamoxifen is crazy for hot flashes and the Arimidex looks bad for joint pain.* Arimidex contributes to osteoporosis; tamoxifen doesn't. Both may cause hair thinning/loss (oh, joy! - although that hasn't happened after 6 months of tamoxifen).
If I could reduce my risk of recurrence to a comparable degree using soy, I'd do it and take neither med. There are a couple of studies out of China suggesting that women with high soy intake get no further benefit from tamoxifen, but there are uncontrolled factors like previous diet and soy intake ... it's not clear that those results would translate to North American women.
*eta - 'looks bad' is an understatement - couldn't sleep last night, woke this morning feeling like I've been trampled by a horse. Now my shoulders, back, knees, and feet ache as well. Aleve in prescription doses doesn't touch it. I suspect the decision is being made for me - the question will be whether to continue tamoxifen with its various risks.