It depends on the bacterial strain used, Measure OD after inoculation of LB broth and when you get on medate and heavy turbidity,centrifuge and remove cells free supernatants
For a bacterial broth culture of 1-50 ml, you can simply get one loopful of your bacteria from a plate and inoculate to the broth. Incubate for 4-6 hours, 150 RPM, 37'C. It should be turbid by then - measure OD, and/or spread on agar plates for CFU counting. All the best!