Once you subscribe through patreon the credits will be given to your account inmediately (or with a little bit of delay), after that you can cancel your subscription (In patreon) right afterwards.

The credit system works independently of patreon, which means once you recieve the credits, they will not be revoked if you cancel your subscription in patreon (Tho keep in mind that they will expire after 2 months).

So yeah, you don't need to subscribe every month, you can just subscribe when you want to get homework critiqued, get the credits, then cancel your membership inmediately after, just make sure that you do get the credits before canceling the membership.

Ledvi21's answer is correct. You can pledge on patreon, receive your credits, then cancel in the interim. Then when you need more credits, you can pledge again and repeat the process. Using this approach, those students with more flexibility in their budget can maintain an ongoing pledge (and allow some credits to expire), while those who have limited budgets can pause/resume their pledge as needed.

This way we're able to take a loss on the feedback we do provide (by which I mean we pay our teaching assistants more than what students pay for the corresponding work), allowing those who maintain ongoing pledges and allow credits to expire to subsidize those who can't.

No need to worry, I completely understand your concerns about subscriptions. The great thing is that, at most online learning platforms, including this one, you typically have the flexibility to subscribe and unsubscribe as needed. It's all about finding what works best for you. So, when you're ready to get your homework critiqued, you can subscribe, and when you're done, you can unsubscribe.