With the current system I could level up Alchemy to 100 under 1hour.
Without cheating? Doubt that.
Alchemy XP is awarded based on the value of the potion created. The only way you could get to 100 in less than an hour would be if you put on a bunch of Fortify Alchemy gear and then did the Fortify Restoraton potion loop, and started making potions worth TONS of gold.
I know this because I've been grinding Alchemy hardcoe lately (without using the cheat) by buying ALL of each alchemist's ingredients, using their alchemy table, and then selling the potions back to them. I've done this a good 15 times over th course of FIVE HOURS and I'm only to an Alchemy skill of 57. I've also spent hours looking for Paralysis, Slow, Invisibility, and Damage Magicka Regen ingredients because those potions get me the most XP... Finding the ingredients alone is time consuming.
If you just collect tons of the really common ingredients, like Mountain Flowers, then the potions won't give very much XP and your Alchemy starts to level very slowly.