I looked. This may have been brought up before, but I couldn't find a topic on it specifically, and I didn't see it elsewhere, though, it's probably there.

I was completing a mission for the the thieves guild and decided it would be far easier if I were invisible. So I dropped by the house, made a few and off I went. I started using them as I got through the mission, but then they became less effective. In fact, after my fourth or fifth, they seemed to last about 7-10 seconds rather than 30 as they were supposed to. Now, I didn't count it out, but is that the case with Invisibility potions? As you take them, one after another, they last less time? Is there some cool down to that?

Maybe it's a bug, but it's happened to me on more than one occasion.

What is your alchemy skill at? You possibly just might be making less potent potions at a low level.

I just use an Invisibility Potion for the first time this morning. The one I wanted to get around could still see me, but they were aware of me before I took the potion (not in direct line of site, but the NPC had talked to me). Anyway, I could get past him even though he told me to get out. So, I am not sure if the potion is working as it should.

However, I do remember from Oblivion and Morrowind that as soon as you did something while invisible, essentially activating anything or drawing a sword or picking a lock etc, then the Invisibility went away. Perhaps this is what is happening to you.

They were all exactly the same potion. I'd made them and they stacked. My Alch is around 40, but I was using three enchanted items to buff it. The potions lasted 26 seconds each "according to the bottle". I also noticed that attacking definitely made me visible, but it seemed opening doors was fine. However, if you go through a load screen, invis would end.

I'll double check and see if I can open chests, etc while invis. That may have been the problem, but I recall opening a gate and still being invis until I went through a load door... but my memory isn't perfect. I will check again and update.

That still aplies, using anything makes you visible. Additionally NPCs can still hear you moving. So sneak slowly, far and naked to avoid detection

boots with a muffle enchant. But I'd swear I opened that gate... I'll look into it.