Around 20-25 minutes, but the route is far less obvious. If you do go though:
Walk out of the station, down Cassiobury Park Avenue, turn onto Shepherds Road past Watford Boys', turn left onto Rickmansworth Road and cross at the first opportunity at the traffic lights. Turn right onto Harwoods Road (first road) and walk all the way down, past the Tesco Express, all the way down through the wreck that is West Watford. Once you get to the end, you're practically on top of the stadium.