Doctors give trusted answers on uses, effects, side-effects, and cautions: Dr. Homsi on how long does it take for valium to take effect: If you are referring to roxicet ( oxycodone/acetaminophen) tablet, then the approximate onset of action is 30 minutes. I take 10 mg of Valium, but only when I have a severe bout of anxiety. After I take it, it takes some time to kick in, but remains much longer than Xanax. Xanax kicks in much faster, but exits your system much sooner. Therefore, I personally prefer Valium. Having said that, if I need something to kick in immediately, I take Xanax. Taking Valium on an empty stomach can help it kick in faster. Hello Debbie Ellis. Valium/Diazepam (long acting/lasting benzodiazepine) will take 1 to 2 hours to begin to work. Age is factor in the drugs half life. I'm wondering how long it takes before you start feeling the effect of diazepam ? I can't really tell, even after taking up to 40 mg's. Thanx for any help available. Xanax is a much more quickly working drug that is short-acting. Valium takes longer for the effects to be felt but does last longer, although in my opinion is not as strong as xanax especially considering the highest dose of valium is 10mg which is equal to the second lowest dose of xanax 0.5mg. Question: how long does it take 5mg of valium to start working. my dentist gave me three tablets take before a proceedure. he told me take one the night before to see how well it worked. Sheeesh! That should swipe you off your feet within minutes. So you either a) wanted to know how long it takes 0.5mg of alprazolam (xanax) to take effect or b) you wanted to harm yourself. 20 minutes to half an hour, full effect in one hour. The short answer is that Valium (diazepam) has a rapid onset of action, faster that nearly all benzodiazepines.