One thing to consider... when using the enchanting table, time continues to pass around you. Which means if you are quaffing a Fortify Enchant potion, the timer on it keeps running down while you're using the table. If you take too long setting up the enchantment, your potion might end before you actually enchant the item.
I don't know about this one. I just did this, made a new glass bow and enchanted it with shock to replace the one I had made earlier. The early one did only 10 damage. I figured since I had leveled up since then, I'd see what I could do making a new one. The new one showed 15 points of shock damage when I selected the enchantment. I then remembered I had a potion that lasted 20 seconds, so I backed out, quaffed it down, then went back in. I selected the bow, then the damage, now 17. I then picked the gem. Finally, I went out and renamed it. When it was all done, it showed as doing the extra 17 points of damage. So, I think once you select the enchantment and set the damage, the damage gets set. I don't think it runs out unless maybe you back out of the creation of that weapon. I could be wrong though. Maybe I was just quick.