About 2 months after you induce flowering, if you are using seed than you will want to build it up for awhile too. Some strains take a bit longer than 2 months, some a bit shorter.
how small are you wanting to keep it? most likely you'll be going from seed... to which i would suggest inducing flowering after the third or fourth node develops. from seed this will take about 3 weeks. This way you can keep it short, in height and time.