Doing a monthly subscription service is not that easy with a smart contract because the price of the subscription will fluctuate as wildly as the price of the underlying ether.
For example, less than a month ago, I think the price of ether was under $500.00 US. Now it's over $1,100. That's the first problem you'll have to solve if you want to use a smart contract for a monthly subscription.
The most likely reason you haven't seen an example or tutorial of this is probably that it can get very complicated. In fact, I don't think I've seen a working example of a workable subscription-based service, but I may have missed it.