It is it difficult/time consuming to set up a Drupal site that users pay to subscribe to?
I've just finished writing a multi-tiered subscription system for an online business directory, using Commerce. Without going into too many details (the clients might be watching ;) it took a fair bit longer than a week to complete and sufficiently test the subscription/payment element of the system, and I'm pretty familiar with how payment processors and Drupal Commerce work under-the-hood.

FWIW I took most of my inspiration from the Commerce subscription products PayPal module, which:


  integrates the PayPal Recurring Billing functionality into the Commerce subscription product module.


It's a feature module so you can just chuck it into a site and see what it does (sets up a product type of 'subscription', adds fields to the user and order entities for tracking subscriptions, handles the recurring IPN notification from Paypal, etc.)

If you read through that module code you'll get a good idea of what's required to set up a subscription system using Commerce. If you also need to write a payment processor from scratch then it'd be a good idea to look at the Commerce Paypal module at the same time so you can get a good overview of what needs to be done.

I'm afraid I can't comment on the UC Recurring Payments options as I've never used it.