wow they stack? I feel stupid. What about potions like burden 20 points, would they stack too? More to the point do all potions stack?
You can stack up to four potions or poisons at once. There is a glitch to exceed that but I don't care about that - four is plenty.
The times won't stack, but the magnitudes will. For example if you pop four potions that each do reflect damage 24 x 78 seconds, you will get 96% x 78 seconds.
So a master burden poison would be 45 burden x 180 seconds. If you get four of those into your foe in rapid sucession, he will be burdened 180 (45 x 4) for 180 seconds.
Its easy to stack 3 or 4 chameleon potions and exceed 100% if you want. Or restore magicka faster than you can cast spells. Or restore health faster than your foes can cut it down.
Edit: Oops. I was posting the same time as glargg. Sorry for any redundancy.

Oh, here is a couple of other recent threads you might like:
http://www.gamesas.com/index.php?/topic/1166804-why-do-people-like-alchemy/
http://www.gamesas.com/index.php?/topic/1167545-potions/