chargify itself, not too long. Especially if you do everything their way(we are kind of doing some weird things with it currently). If you don't want to write your own, there are a bunch of API libraries available for a bunch of languages as well as hosted paged for registration and payments if you need them. As everyone else has said, the long parts have been the up front setup time of the merchant account and payment gateway.
All in all though, I'm pretty impressed with Chargifys offering.
Its just that much of our billing system was done before we ended up switching to chargify, so some of integration pieces are kind of weird(probably not how we initially would have designed them) and we are looking at doing some things with quantity based pricing, where we do something similar to linode and charge you a prorated amount up front for a item then charge you the full amount during the next period. As well as offering prorated credits for removal of a product.
This is not supported by Chargify yet, they only charge for items at the beginning of a period. If we were using a simple recurring subscription for this and not quantify based model this wouldn't be an issue, because they do offer prorated upgrades and downgrades for recurring subscriptions.
Nothing overly complicated here, just have to hack around some of the current limitations of the platform.
All in all though, I'm pretty impressed with Chargifys offering.