From what I’ve read in other threads; for smaller jobs the Glowforge is able to download the slicing in one ‘chunk’ and then (this is my speculation now) send periodic updates to the cloud portal so you can see how much it has done / how close it is to finishing. For larger jobs though it breaks this up and downloads them piece by piece as it goes through the design… I’ll see if I can find the thread I read this in but hopefully Dan or someone from the GF team can come through an confirm.
So in answer to your questions:
Does it have to stay connected - unsure (I believe this would depend on the size of the print)
Does it connect send/receive and disconnect - unlikely given it’s sending out update; but losing connectivity shouldn’t affect the print unless it’s being done in multiple ‘chunks’
How much data is used on average - The glowforge has the ability to take a video (timelapse?) of each print so this would affect the amount of data used; if they’re using a gcode type instruction set then most print jobs should be pretty small, in the vicinity of 20MB tops?
Glowforge; please vet this for mistakes and let me know if I’m wrong 
Update: Found this on another post [quote=“dan, post:4, topic:138, full:true”]
You do need a wifi connection - if it drops, the print will finish (or pause if it’s long) until it reconnects.
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