RE: "Glassing"
Sorry to be a wet blanket on your above idea as much of the the fun of this hobby is experamintation.
As to your core question, I'd use 30 minutes which gives you more time to wet out the cloth thorughly and blot our the excess epoxy with paper towls. (The real steanth is in the glass cloth, not the expoxy.) To get the epoxy a little thinner like you said you wanted it, you can heat the expoxy slightly by putting the bottles in hot water or hitting the mixture briefly with a heat gun. That's an alternative to adding a liquid thinner (which I've never done so I'll be listening to other replys). Epoxy "goes off" faster when mixed in a cup than when allowed to spread out like on a piece of waxed paper, but either will work fine.