In this study, a significant decrease in bacterial flora was apparent after just 48 hours and almost all the bacterial flora was wiped out within 5 days. One case of death was noted due to antibiotic-resistant strains. This was all due to needless prophylactic dosing - http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2134008/?tool=pmcentrez


Another similar story on the effects of antibiotics - For nine months after exposure, the subject’s gut was left with nothing but one type, a clindamycin-resistant strain of Bacteroides thetaiotaomicron. Even two years out, the flora had not regained their former diversity.

http://www.wired.com/magazine/2011/09/mf_microbiome/