So I'm doing a long trip later today, need to check the oil. Car's been parked on the driveway overnight which slopes so I can't do it there (obviously it lies) so when I went out earlier today, I parked the car up the road on a flat part. How long does it need to be there before I can get an accurate check?

couple of minutes max

Yes it really shouldn't take long before becoming checkable. When you add oil it hits the sump relatively quickly so pretty much as above really.