Given the problem you've posed here, I think that it would always be a 30 card deck, but I'll freely admit that this is based on an off-the-cuff educated guess based on my knowledge of statistics. Perhaps someone (probably not me, but who knows?) will try this out with xeno's simulator, as it is exactly suited for this type of task; no-brainer decks playing against goldfish opponents. As long as you can reach an optimal quanta supply with only 30 cards then 30 will be the best choice.
http://elements.alanbeam.net/qi.php?deckCode=808+808+808+808+808+808+808+808+808+80f+80f+80f+80f+80f+80f+80g+80g+80g+80g+80g+80g+81q+81q+81q+81q+81q+81q+81q+81q+81q+8pu&showImage=1This is the code and QI calculator for a 6 dragon 6 recluse 18 (upped) quanta deck, and the QI is 4.78, which being under 5, is probably the best scenario. Assuming that a QI=5 is ideal for this deck (it may not be; this would require simulation/testing) then the ideal number of quanta is actually a deck that is smaller than 30 cards. Putting 6 dragon 6 recluse 17 towers into a QI calculator gives QI=5.05. If you were to use less towers (and again, assuming QI=5 is ideal here, which may or may not be true) then the deck would actually become slower, because you'd be quanta starved. Take the extreme case of 6 dragon 6 recluse 1 tower as a 13 card deck and you'll see what I mean... you won't get very far with that :p
Basically, what I'm saying is that as long as you can get the ideal QI in a 30 card deck (or less, but are forced to use 30 cards to get that high) then 30 is the best deck size for a rush.
Your question would have a lot more meaning and discussion if you added one more card to it, since in this case I think the ideal is almost certainly the 6/6/18 split I mentioned above. If you were to ask instead "say you have a deck with X upgraded phase dragons, Y upgraded phase recluses, *and Z upgraded aether nymphs*, with the rest of the deck being aether towers, what is the fastest rush deck, assuming you play against a dummy", then the question would get a whole lot more interesting.
edit: my point still stands, but some examples were posted while I was creating this post, which should hopefully further illustrate what I was trying to explain.