The Poirot books take readers through the whole of his life in England, from the first book (The Mysterious Affair at Styles), where he is a refugee staying at Styles, to the last Poirot book (Curtain), where he visits Styles once again. It actually covers the 20's 30's 40's and 50's!
The ones with David Suchet are generally set throughout these years depending upon when the original book was written, but the timeless Poirot 'image' is probably from the 20's.
The tv version was firmly set in the 30's dont you agree Octavius?. They went to great pains to place Poirot in most of the iconic buildings of the period. Cant speak for the books cos ive never read em' and i only watched the tv show to spot all the nice buildings.