The answer to this question depends on two factors. First the type of wood that was installed, and two, the area of the country that you live in. Some pressure-treated woods have wax in them that helps protect the wood from water damage. They can withstand the elements without any protection for a full year. If the wood does not contain wax, the length of time before you should apply the protection is based on the area in which you live. If you are in a dry area, then I would suggest a wait of no longer than two weeks. If you are in a more humid area, then you should delay the application for at least a month, and maybe an entire season. The best way to find out if the wood is ready for water protection is to pour some water on the deck. If the water forms a puddle that remains, then the wood is still well-protected. If the water seeps into the wood, then it needs protection. With regard to staining the wood, if you do not first stain the wood, over time the color of the wood will turn gray. Even if you do stain the wood, it will still turn gray over an extended period. Water sealers with ultraviolet (UV) filters will slow the discoloration of the wood.