Of course it will all come down to if I feel I can handle a long one when the time comes.
"Ooooh, Matron!"
You should be so lucky, Pete!
As great as Once Upon a Time in the West is, I wouldn't start out with it. Start with The Big Country, it had a good story and it's not too long.
I know Pete already said this, but I did wonder. The Big Country is Massive! I recommended Once Upon A Time In The West because it's possibly the most cinematic film ever made, so although it's lengthy, it flows beautifully and is a unique experience that everyone should dive into. Achim's post perfectly sums it up! But maybe if you are still worried about the time, Pete, you could consider one of the other Waynes, like Big Jake or The Cowboys. You'd enjoy those after Rio Bravo.
I adore The Searchers, but I avoid recommending it to Western newbies, because I think it works better when you understand what a typical John Wayne western is like and you then see why The Searchers is a different beast.