My remarks are based on a fair amount of reading. If anyone has info to the contrary please let me know. (And I'm sure you will).
It should be noted that different types and batches of the same type of mushrooms can have radically different potency (take a look at the Mushroom FAQ on HyperReal to see what I mean).
Also, there is great variability from person to person. My girlfriend needs higher doses than I (fyi, I outweigh her by forty or fifty pounds)
The dosage that S. mentioned (27 grams fresh should be the equivalent of almost 5 grams -- see note below) seems enough for a moderate to heroic trip. This amount of the mushrooms I have available to me would have left me silently and ecstatically gaping into the face of the void. The amount for the second trip (15 grams dried) would have been way beyond the max dose for me anyway (see notes on Tolerance below).
I suspect that these were from a very weak batch. I can personally attest to the fact that some batches of p.c. can be more than twice as potent as other batches. I would ask the source what the recommended dosage is.
(The last ones I tried launched me into hyperspace with 2 grams and gave me a stronger trip than 3 grams of an earlier batch.)
The first trip definitely would affect the second (see notes below). But, I suspect that either these mushrooms were super weak or not p.c.. OR that your brain has an interesting serotonin mechanism.
Fresh vs. dry dosage
Even though fresh mushrooms outweight dried by about 10 to 1, the difference in dosage is more like 5 or 6 to 1. The reason is that fresh mushrooms contain almost equal parts psilocin (which breaks down completely when they are dried or heated) and psilocybin (which, of course, is turned into psilocin after ingestion). So, the same mushrooms are about 1/2 as psychoactive after drying as before.
Note that there is fair variation in the content of mushrooms. So, not all p.c. will have the same ratio of these two chemicals.
LSD, psilocybin, and mescaline all build up tolerances incredibly quickly. Scientific studies have shown that if you give the maximum dose of LSD on successive days, it is completely inactive by the 4th day. Psilocybin and mescaline have apparently the same response/tolerance curve.
Also, even before tolerance builds up, there is a maximum amount of these substances which the body will even use (which is why they are non-toxic). So, taking 10 grams may affect you no more than 6 or 7 grams. What this point of diminishing returns is varies from person to person.