You are wise to bring this up: stress cracking is an issue I was worried about in the design phase - getting glass above the stress temperature (about 950 F for boro) repeatedly is definitely hard use. This is actually one of the reasons why the vapes are only 1.5-1.8mm thick for the heat exchanger section. Thick glass doesn't deal with repeated heating and cooling very well - basically the outside of the glass cools faster than the inside, which makes a layer of glass that's "set" in position before the glass on the inside is done contracting, and it cracks trying to shrink to the room temperature position. This is why all the glass that comes out of the shop has to be annealed - held at a stress relieving temperature and then cooled very slowly past the strain temperature - before it's ready to use.
Once the glass is a certain thickness, however, it can cool evenly enough that stress can't build up to a very noticeable degree. Borosilicate glass nails for oils usually don't work for very long, since they need to be thick to retain enough heat to function - eventually they crack due to the stress built up from repeated heating and cooling (especially the flash-cooling induced by adding the oil). What I've discovered from making and using these vapes for quite some time now is that the low wall thickness combined with the rippled design yields a piece which can take a lot of thermal abuse. I have had a classic heat exchanger in constant use as a demo model in a head shop for a month and a half, with 20+ uses per day, with no ill effect - both bowl sections were black with burned on tobacco, but the piece never cracked.
This is also one reason I only use Simax (and sometimes Duran or Pyrex) lab glass for anything clear (and especially anything exposed to heat repeatedly). I've found Chinese clear to be especially prone to thermal shock, to the point where I don't even bother having it in the shop. I've wasted far too many tubes of glass by putting the tube in the torch flame and watching as a foot of raw glass spiderwebs cracks and falls to the bench. Chinese ground joints obviously won't do for the heat exchanger of the GonG vapes, so quality joints have to be sourced. It's not worth it for me to be cheap, for me or for my customers.
So far I've only heard of one of the vapes I've made cracking from stress after months of use... I can only think this must be a defective piece, since so many others have been in use far longer, and I'll happily replace heat exchangers that crack within 60 days from purchase, provided you send me a photo of the cracked piece showing the stress crack. (under no circumstances should you actually send the piece back to me!) After that period, please contact me and we can work something out. I will also sell replacement HX or mouthpiece sections separately in case your vape has an accident that leaves it partially intact.
All that said, heating the rippled section evenly greatly helps with peace-of-mind when it comes to the long term durability of your glass. Blasting it from cold in just one spot is less safe than moving the flame slowly across the surface of the exchanger, and heating the half of the exchanger closest to the "bowl" works really well if your torch has a teeny tiny flame.
Hope this has been informative, if a bit long winded!
-gn0me!
It takes about the same time as with my previous torch.. When tank is full less than 30 secs for sure. 
