Not from GF, but I’ve been reading everything on this site for a long time. I think these are the answers you are looking for.
Same thickness as anything else, 1/4 inch per side. 1/2 if you flip it and re-cut the image on the back side
No specific hours given, but an estimation of “normal” use has it at about 2 years of use before needing to replace the tube
I’d advise to do some research on its safety. I remember i looked into this and i decided not to use it (at least on a regular basis) due to its formaldeyde contents.
The other thing you asked is can it cut mdf, yes and 1/2" in two passes, cut one side flip it and cut the other.
And welcome to the forums.
MDF varies heavily based on glue used and density it is packed to. So you could possibly find a source of MDF for which even 1/4" was not possible on the Glowforge. But… most distributors probably supply MDF which would cut very easily at this wattage. (note: unfounded opinion. I have minimal experience with sourcing MDF)
I know it sounds condescending, but I mean it as a precaution: Go read the specs on the machine. It has been a year since you ordered, so you may have forgotten exactly what kind of limitations you are working with. 4-5 inches won’t even fit in the machine, so it cannot cut that thick of ANY material.
But… you can do a lot with glue and layers. So it is possible to build a 4-5" thick box.
I’ve not found any domestic (U.S.) MDF that wouldn’t cut with a 40W laser. I don’t use it for a lot of stuff (usually just some set props) so I get it from a local big box building supply store. If you’re using it for a lot of stuff, there is laser MDF that is certified formaldehyde free and glue safe. It gets a bit dirty using MDF a lot. Also with the GF it might lead to early filter clogging as it is a dusty mess.
From the “kerfuffle” thread:
The more I work with MDF with my CNC the more I hate it. I’d love to make some custom speakers but if I was going to go to that much trouble I think I’d use baltic birch plywood or from solid wood.
Alright guys, appreciate the fast reply! But how about the plywood thickness on one side?
Quarter, half by flipping.
Is that a product similar to TREX in texture?
The problem with using “real timbers” for speakers is the lack of uniformity and how it can effect the sound produced by two, seemingly identical, boxes. Veneered, or painted, MDF is a much better proposition.
I’m sure you are right from a consistency basis, I just hate working with it. Perhaps concrete?
That may have been a bit tongue-in-cheek but when I was a very small lad, in the ‘60s, my father bought a kit to fit large Wharfedale speakers onto the top of 4’ x 2’ concrete water/sewer pipes. The inside was filled with baffles and wadding. True story!
I couldn’t find an image to show you but there are references to the system on the web.
No, not all that tongue-in-cheek at all. Would be at least as much of a pain to work with as MDF but the acoustical possibilities! Definitely not for a nomadic person though.
You could add sawdust or perlite to the concrete mix to make it lighter.
Concrete is popular in car stereo competition for damping behind body panels. Yes, the wet mix is poured right into the cavity between the outer and inner panel! Making the damping material lighter defeats the purpose, the process is referred to as Mass Damping because of… well… Mass. 
On the flip side, I recently built a subwoofer enclosure using carbon fiber. The enclosure weighs about 5lbs, the driver installed into it weighs 45lbs LOL… it sounds beautiful! Granted it’s all frequencies below 80Hz.
Acoustics was once a part of my career but I have no idea how to predict resonances. Something I’d have to study up on before building speakers.
I do understand that mass generally works though.
I am curious what kind of sound damping you can get with aerogel encased between layers of carbon fiber…
I have had very good experiences cutting mdf and have not gotten any that didn’t work on a 60W laser (2-6mm).