Antimicrobial agents active against mycobacteria have to be administered for months or years rather than the typical 10-30 days prescribed to treat other infections because mycobacteria reproduce very slowly.
So, over the period of time that mycobacteria reproduce, antimicrobial agents have to be taken so as to prevent these mycrobacteria from invading the body completely. Once their reproduction is completely prevented, antimicrobial agents don't have to be administered anymore.