You can achieve that with 2 differents method, choose your preference!
Method 1) Fixed Duration as VARIANTS/OPTIONS on product :
Method 2) Fixed Duration with DAYS ALLOWED & DISCOUNTS :
You go for single (or range depending of your preference)
enter the fixed days you want :
Now you just have to use discounts based on the amount you want for each duration!
let's say you want these prices :
4 days = 300$
8 days = 600$
12 days = 800$
You just have to set your product price (on Shopify product edit side) to your first price (here 300$)
Then just use the calculator for get your prices :