A time saver too far?

When out shooting Roseberry Topping from Gribdale Terrace I ended up deciding to do something that I never normally do - and now I'm not sure how I feel about it. I replaced the sky in the image... but there are some qualifying remarks to go with this, which leaves me wondering just how 'okay' it really is.

Original on the left, edit on the right. 

The sky above Roseberry Topping was so lopsided... clouds to the left and empty to the right... and I had a panorama I really wanted to get shooting from a few yards to the right. So, with a choice to make and a limited amount of time, I decided that I could just move time on myself on the computer. The clouds were going left to right across the sky and so if I'd just waited 10 minutes in my original position, I'd have been able to capture an identical photo to the end product here... but I just didn't have that time (or I need to buy an extra camera to do both at once!). Really, I did see this view as I was shooting the panorama, the clouds did what they should and filled in the space nicely, it's just it's not technically the sky from the same image - it's the sky about 30 degrees to the left, or about 10 minutes too early. 

I really love the final image and I'm reassured by the fact that I did have the opportunity to shoot exactly that view a few moments later had I not been busying myself with other things - but it's left me wondering a few things. For example, how is this different to if I'd done multiple exposures a couple of minutes apart? How many minutes apart must exposures be before it's cheating to use the sky from one on a differently timed foreground shot? Do they have to be immediate? Can there be a minute between them? 10 minutes? An hour? Does it not matter unless the light has changed? It's not something I'd thought about before so I'm not sure of where I draw the line personally. 

I know there's a lot of discussion about honesty in photography, and I hate being misled by images, which is why I felt compelled to write this post to at least mention the editing of the photo. But I don't think it's ever really discussed regarding the point at which it becomes necessary to admit what you've done... It's difficult to draw a line between what's passable and what's not.