How long does it take for Valium to take effect?

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. It is effective orally within 20 to 30 minutes. Its peak plasma (blood) concentration occurs between 30 minutes and 2 hours after administration.


For the longer answer:
Diazepam can be administered, orally, intravenously, intramuscularly or as a suppository. The onset of action is 1-5 minutes for IV administration and 15-30 minutes for IM administration, although onset can be faster. Diazepam's peak plasma concentration is 15 minutes to 1 hour for the intravenous or intramuscular route.
Diazepam has a half-life of 20-50 hours, it has several active metabolites most notably nordiazepam, also known as desmethyldiazepam (average half-life 30-55hrs), temazepam (average half-life 10-40hrs), and oxazepam (average half-life 5-20hrs). Diazepam is a long acting benzodiazepine, in some cases diazepam's active metabolites my last for up to 200hrs. For this reason, it is often substituted for other shorter acting benzodiazepines when dosage reduction or withdrawal is desired.