>>sleepy, the instructions on most packages of food are designed to get the consumer to replace or feed more frequently, thus increasing sales. look at the packaging on betta pellets for example, no wonder most people's fish die so soon, it's usually from overfeeding/not keeping up with the cleaning of the excess waste produced. in the wild, cresteds eat rotting fruit, which i'm sure is loaded with bacteria, some of which are beneficial for their gut. the geckos only need to be fed 3 times per week, so as long as they are getting fresh food that often, it is not harmful to leave the CGD in a little longer between feedings.
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Ummm... Really? Now, I can't say anything about your fish example because A.) I don't keep fish and B.) Not really sure where you were going with how overfeeding vs offering fresh food.
But hey, I've been up for almost 24 hours, so let's see if I can put together a logical argument everyone will understand. 
Ok, just for now let's say cresteds do eat rotten food in the wild. (Not all animals do, I haven't personally read whether cresteds are fresh only animals, but contrary to a lot of common thoughts, most animals can tell the difference between healthy and unhealthy food and will toss the bad stuff, this is well researched.) OK, but then we are still talking about naturally occurring generally WHOLE, but rotten, fruit. Meaning only the outside of the rotten fruit has been exposed to air and oxidized the nutrients.
Also the nutrients occur in nature in special combinations to be appetizing and healthy for the animals that eat them. (Why? Because if no one eats them the seeds don't get spread around.) The balance of the nutrients serves to keep the food as nutritional as possible, because if the animal that eats your fruit is the healthiest, then they will spend the seeds far and wider than a sick animal. CGD is a complete supplement, but at least some of the compounds are artificially created so we don't have those carefully balanced tradeoffs to prevent degradation. There are two options, add preservatives or shorten the "activated life" to keep the nutritional value more consistent.
You'll notice the package in the bottle, to prolong the shelf life of the product. When we mix it with water and expose it to even more oxygen the clock is ticking very quickly. Even human vitamins contain extra ingredients to prolong shelf life or are coated to prevent oxidation and other degradation processes.
So, in summary, I am not saying it is some horrendous thing to leave the CGD in the tank for long periods of time (unless it molds) however I am saying that that nutritional value drops so there is a very good chance if your gecko is still licking up the diet on the 3rd or 4th day (which I have seen) they are getting food and the feeling of being full, but not the nutrition... like eating puffed rice cakes or something. (might be extreme, but you get the idea)
Really, MARS by Ben Bova is a great sci fi novel to show just how important handling nutrients is in an artificial environment. Which, for all purposes, keeping our cresteds in a tank is not much different than keeping humans in a tank on Mars.
(Oh, and I'm still going to reiterate that the crickets should be dusted with a complete formula as well in order to balance the nutritional needs of the gecko. This has been stressed to me too many times for it not to sink in. Calcium = good, Calcium w/ other stuff = better)
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0.1 Albino Leo Gecko
0.1 Crested Gecko
1.0 Dear Boyfriend
Departed: Harvey and Spock
