The reason I made Session is for the analytics and syncing between devices. I want to start and review my progress on any devices I currently on.
If what you need is only changing the time chunk—I think Session is not for you. There's a lot of excellent product on the market that's cheaper or even free for that functionality.
My question: have you tried Session? If so, what do you like about it?
As context, I dislike subscription and prefer one time purchase as any of us. But that business model is proven to be not sustainable—lots of great products that I use died in matter of years because of lack of update. iOS update means breaking change to the API and could introduce new bug. Ideally I would love to sell the product as one time payment with free update for a year. But that's currently not possible on iOS...