Time does not stop while at the enchanting table.
I have been able to do 2 enchantments with one potion but if you add in a renaming of the item, which I wanted to do, then you simply do not have enough time to really do more than one item.. Just the length of time the UI takes to select all three pieces is about 15 seconds in and of itself..
My suggestion is to just always play it safe. If you have enough money on hand to be trying to super-enchant your items, then you should have no problem finding the ingredients at the various shop keepers. I make a loop through Whiterun, Solitude, Windhelm, Falkreath and Morsomethingorother (I can never remember the name of this one!). Takes me a few moments, costs me maybe 1000g each to clear out their inventories and I can always make the set of potions I need to do whatever crafting I am up to plus enough misc ingredients to more than make my money back if I do not want to get more gold via enchanting :)
As for your final questions, 30 seconds has been fairly accurate to me. It is real time, as long as the game is not paused (If you have your inventory open or the like it wont tick down the duration, if you are in the enchanting table though it does). Honestly its the slowish interface.. I believe they timed these potions specifically to get you a single guaranteed enchantment, and then maybe 1 more if you are just rushing it (But since the price doesn't really go up at these levels its rather pointless to be rushing these :))
More Info: Here is probably one of the handiest websites relating to Skyrim for crafters. http://rp.eliteskills.com/skyrim.html This is a potion calculator that is very easy to use (Click the Add All Components and away you go) and will allow you to find what possible combinations can make the potion/poison in question, how many ingredients you want to use and as luck would have it, is always focused on showing you the most expensive first (So when crafting for skill/money go with the top, when crafting for use go with the bottom :)).You can use this to get rid of all the extra ingredients and gather your money back on having to buy the more expensive items.
And now a video:
I enchant 5 daggers to give them level 26 banish instead of level 25. Just watch the timer of the video, it is not bugged.