Non-time limited credits

I’m a student and so don’t have a lot of free time to work on my story. Sometimes only once or twice a month. During such months (which are most of them) I can’t justify to myself paying for a subscription when 95% of it would go to waste, so I end up skipping on using the site despite really enjoying it and how it contributes to my writing process. If there was some kind of non-time limited credits, even if it cost more per credit, I would use it in a heartbeat.