There isn't an absolute benchmark that says "doping" unless it is a world record today. Below that involves an assessment of the athlete, their progression, where they come from, who trains them etc. However, what can also be taken as a rule of thumb is that with the estimated prevalence of doping in sport today, and that only "the dumb and the careless" get caught, it is unlikely that any athlete can compete at the top of the sport without it. The recent surge in times run by college age teens is also a red flag for me. It isn't just explained by training, tracks and shoes. WADA has categorised T and F with bodybuilding, weightlifting and cycling for risk of doping. What do you think are the chances that the best competitors in those sports aren't doped? A few years ago it was reported that 10 of the 11 fastest times over the 100m were from athletes who at some point incurred a doping violation. The one exception was Bolt - the fastest. Sure.
Because of the role played by doping today I don't think it's possible to give a time that must be considered "clean as a whistle", because a possibly mediocre but national level athlete could attain that time through doping. Perhaps 40 or 50 years ago such a time was probably assured to be clean.
My perspective is essentially this, that the presence of doping in sport (reliably reported as a billion dollar black market that mostly can't be detected through testing) has meant no performance can be trusted today, whether it is pro, elite, masters and even in schools.