Each genre has its own parameters for 'show-and-tell' and no two such measures are alike. Each editor in each genre has his/her own preference for show-and-tell balance in the book he/she's editing. I've known this for a long time but had it reinforced just 2 days ago when I received my first round of edits on my upcoming YA novel. Keep it moving, get to the point, kills your darlings (prose that is), etc, etc.
But there is a quick rule-of-thumb. Most people - readers that is - have low tolerance for great chunks of prose crowding the pages, whether these be electronic or paper. Long prose is off-putting, to the young and old alike. The editors tell me this and it's actually a reflection of what I feel at times when I stare at a densely populated page with words and words - without dialogue to break it up and catch my breath.
For that foremost reason, the writers are encouraged to 'balance' their writing craft such that you will have no more than 30-35% of any given page as exposition (dense prose) and the rest would be dialogue. It's generally good balance that makes for a tolerable read and if the prose is interesting and the dialogue is dynamic, it makes for a compelling read.
As a reader, I have an exceptionally low tolerance for pages of pretty but meaningless prose, describing details that with each turn of the phrase make it more and more unbelievable and therefore building up to the point where I will toss the book in the donation bin at the mall or return it to the shelf. I do have patience and trust that the author will lead me to the point, but it better be done quickly or he/she will not be given a second chance.
Think about this - your publisher will ask you to provide him with a 'blurb' for your book that should not exceed 150 words and perhaps not even exceed 100 words so think of balancing your written prose with that rigid stipulation in mind. Getting to the point quickly is important therefore you must learn to "prose-up" any given page in such clever manner that the reader will stay on it - and turn it to keep reading.