Your father-in-law:

Although cedar is a naturally durable wood ideal for decks, its performance is enhanced when protected by an appropriate finish. Decks have full exposure to sun and rain, which greatly accelerates the weathering process. In fact, so aggressive are the effects of weather extremes in some areas of the country that a deck may need cleaning, restoring and refinishing as frequently as every two to five years depending upon the finish used. Decks should never be allowed to weather before finishing.The simplest, but most labor-intensive, finish to maintain on a cedar deck is a water-repellent preservative, which may have to be applied annually. The next easiest is a semi-transparent oil-based stain. Both types of finishes are extremely effective in stopping the absorption of water and are recommended. It is important to ensure that the product has been specifically formulated to withstand the abrasive effects of foot traffic.
Oil and latex solid-color stains, (also called heavy-bodied or opaque stains), paints, and other film-forming finishes are not recommended.

If there is uncertainty over whether to use a water-repellent preservative or a stain, first apply a water-repellent preservative. It is possible to switch to a semi-transparent stain when the deck needs to be refinished. Even if the deck has been maintained with a water-repellent preservative for many years, an oil-based semi-transparent stain will perform satisfactorily.

Vist the western red cedar association site you'll learn even more about the care of your new deck. http://www.wrcla.org/index.asp

-Gordon-
Alter Eagle Construction & Design

Is it green treated?If it is you must wait for it to weather,if not the green treat will trying to come out when you sealed it in thus making your stain or repellent ineffective because the natural oil and green treat of the wood will be pushing out and in essence pushing out your product you put on.
A water repel like thompson's is much easier for a home owner to redo,a stain if it starts flaking or peeling off now you must sand or pressure wash this off.A water repel type you can easily apply with a hand pump sprayer the stain will last longer but is a pain if it fails.Water repel will last only 1 or 2 years, a stain maybe depending on abuse and such 3-5 years,but 3 times as much to redo,make sure you use a decking stain.
As far as the waiting period is up to you I can tell you this,if i was hired to do it I wouldn't warranty the finish or job if you did it within 3 months of being built.new wood does not absorb very easy (green treat or natural.