How long should treated lumber dry before applying water sealer?

Usually 90 days but a great deal depends on where and how you are storing the lumber and also how wet is was when you first bought it. If this is already built into a deck or exposed surface I'd go 90 days and no rain in the last 5 days. Most sealers will give you their own details on what they recommend. I'd recommend Thompson's Advanced or their Water Seal Plus Wood Protector which comes clear or in 4-5 tints. THOMPSON'S 11806 WOOD PROTECTOR - 6 GALLON