Taleweaver is giving awesome advice.
Most professional VNs only have 20 or so locations or backgrounds, quite a few significantly less. Most of those are the same background at a different time of day. As a professional artist, even I am constantly revising my game script and design based on amount of workload. Like was pointed out, adding a single new character adds at least 4-5 more sprites. And do characters change clothes? Add another 5 sprites.
You'll quickly learn to start thinking more conservatively: "Does this scene really need to be in a new location? Wouldn't it work just as well in an existing location?" etc. It is best to plan your backgrounds so you get maximum use out of them, making sure each background is used as often as it can be. Do you need a school hallway at all if you have a classroom background, for example? Your characters can talk before or after class without leaving that room in many cases. The walk through the hallway is implied.
And as an artist that has been doing this a while, both as a hobby and as a professional, your time estimate for the project is always going to be wrong. It will always take longer. A project always takes twice as long as I project it to. So if your friend says she can do one sprite every 4 hours, it is more likely each sprite will take 8 hours. This formula - take the projected time everyone agrees and thinks it will take, and double it - holds up very well. I worked at a studio doing special effects for movies with budgets in the millions and professionals everywhere (we're talking James Cameron movies and the like), and we would estimate the time it would take us to complete a shot at say 3 weeks. It would end up at 6 weeks. "Hmm, this shot looks easy. 1 week. Everyone agree?" 2 weeks is what it took. I even had one shot that my team leader and I swore would be a 3 day job and it took us 4 WEEKS. The moral of all this is that even with a professional team that has done everything before, a proven production line, and millions of dollars, everything still takes longer than they think it will.
And yes, most artists would balk at doing 50-70 BGs. Backgrounds take longer than sprites, so if you friend says 4 hours a sprite (which we have already established will take longer), lets say she estimates BGs to take 8 hours. This probably means they'll take 12-16 hours apiece, but lets assume she is the most accurate artist in history at estimating how long she'll take on something. 70x8 is 560 hours. 50x8 is 450 hours. 20x8 is 160 hours. Or, to put it in more visceral terms: 70 BGs equals 23 whole days of her life, 50 = 19 whole days of her life, even 20 equals 1 whole week of her life. Just for the backgrounds.
And 26 characters? Let's say she is only doing one pose with only one expression for each, at her original 4 hours estimate. That 104 hours of her time.
I hope you are paying her WELL, because at my rates (which are cheap for professional artists), for her time estimates, for 50 BGs and 26 characters with only one pose and expression, you would owe me $14,000 dollars. And if I were being forced to lock into something as long term as your project (meaning I wouldn't have time to work with other clients) the price would probably go up to something like $22,000 dollars. This link to
Canned Dogs blog showing professional visual novel costs is probably something you should look at. It costs them $150 to $500 for each regular BG for example.
So, don't take advantage of your friend.
I think doing something like Papillon is doing with Magical Diary is the best plan for you. Do one dorm per game, with just a handful of characters. Release it and sell it. Then make the next dorm and group of characters and sell it as an expansion pack. Rinse and repeat as many times as necessary, but don't go "whole hog" and try to do it all at once. There is a reason books and movies get released in trilogies or sections instead of waiting 10 years and putting out 5000 page novels or 10 hour movies.