This issue has cropped up many times on the Panda thread too, with usual lifetimes from 20k to 40k. A lot will depend on the stresses it gets.
As a result of the regular threads, I bought a pair of bottom arms 3 years ago, as they were on offer, still not needed them yet. The rubber will probably age harden in the garage before I get to fit them!
The more they flex, the shorter their life. So not just bumps and potholes, but acceleration and braking will affect them. If they've been subjected to lots of late firm braking, they'll fail sooner.
Doubtful you can find any manufacturing defect in the bushes, so it'll be wear & tear.
Drive smoothly, avoid bumps and potholes.