My personal guideline for the 27% #1 solution is no more than one month. Since I normally mix my own solution 10 ml at a time, I don't have much trouble getting through each batch I make up. Louis Nadeau in his Theory and Practice of Platinum Printing says storage up to six months is OK. Definitely, your mileage will vary here. FO is so much less expensive than the metal salts that erring on the side of short storage isn't a great sin.