Is it the stationary phase, as it is reported that it is during this phase that bacteria confer high rates of resistance ?
This is correct that bacteria are much more resistant in stationary phase than exponential phase.
However, I think that the time of adding antibiotic to inoculum depends on some factors such as the nature of selected antibiotic (bacteriostatic or bactericidal) and the fact that studied bacterium is slow/fast growing.
Routinely, for testing the expression of antimicrobial resistance genes, bacterial inoculum should be prepared at logarithmic or exponential phase, in which you have the maximum growth rate of your bacteria in order to measure the susceptibility of them against an antibiotic.
Bacterial metabolism is slow down in the stationary phase due to many other factors (limited nutrients, accumulated toxic metabolites), and part of the population will die in a short time without additional factors. So, the antibiotics effect will be lower.
Don't forget that many antibiotics are only effective on growing / dividing cells. For example beta-lactam antibiotics. So if the culture is in stationary phase and the cells are not dividing, there is likely to be minimal effect by the antibiotic.
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