For instance, if there were a product that takes a variable amount of time to mature, could a business take pre-sale orders (clearly marked as such) without giving the customer a deadline? If so, what keeps a business from tying up customer funds indefinitely without ever delivering?
The answer, as with almost every question about U.S. law, depends on the jurisdiction you're dealing with.
In practice, though, most state and federal unfair trade practices (UTP) laws are based on one pretty basic question: is what the shipper is doing fair? These laws normally take the form of a long list of specifically prohibited practices, then a "catch-all" that basically says, if a reasonable person would think it's a scam, don't do it.
In general, if the risks are disclosed to the customer, and the seller is acting in good faith, the mere fact of a long lead time will probably not be in violation, although of course state laws will vary; some may require refunds under some circumstances. If there is inadequate disclosure, or the seller has no intention of selling anything, then it will almost certainly fall under the state UTP law.
In other words: if your unicorns only lay eggs every couple of blue moons, you can sell the rights to the next egg as long as you fully disclose this to the customer and, possibly, offer a refund option if your state requires it.
If you're selling the rights to your unicorn eggs but you don't actually have any unicorns, you will almost certainly be in trouble.
Mail order sales are a special case, governed by federal law. The FTC has a faq for non-lawyers on compliance with the law. But, again, all they require is that you are honest with your customers. There is a section specifically dealing with "dry-testing," basically, selling a product that is not yet available (and that won't be if not enoiugh people order it). Here's what they say:
In an advisory opinion, the FTC told a publishing company that it could "dry-test" its merchandise as long as the following conditions were met:
In promoting the merchandise, the merchant can make no suggestion that the merchandise will be shipped or that customers expressing an interest in it will receive it. In all promotional materials, the merchant must disclose all material aspects of the promotion, including the fact that the merchandise is only planned and may not be shipped. If any part of the promotion is later dropped, the merchant must notify subscribers of the fact within a reasonable time after soliciting their subscriptions. If, within a reasonable time after soliciting their subscriptions, the merchant has made no decision to ship the merchandise, it must notify subscribers of this fact and give them the opportunity to cancel and, where payment has been made, make a prompt refund. The merchant can make no substitutions of any merchandise for that ordered.