For how many years does a u. S. Senator's term of office last before his/her seat undergoes another election?

2 years

Explanation:

The requirement for senator's reelection is regulated under the Article I, section 3 of the untied states constitution. The senates seat can be filled for the maximum of 6 years, but the senate have to undergoes re-election every 2 years. But, for each election, only one-third of the senates are eligible for the race in order to maintain the stability of the senate office.