This leader has only been in power for three months.
Tips
usage
上台 has two distinct uses: (1) physically going on a stage to perform or speak, and (2) metaphorically coming to power/taking office. The political meaning is very common in news.
usage
The opposite is 下台 (xià tái) — to step down from power, or to leave the stage. 下台 can also mean 'to be put in an awkward position' (给某人下不了台 = to embarrass someone).