Taking draughts in overlapping stages increases the regeneration ability from that point onwards and shortens the duration, but time already used counts towards the time limit. So, if a character took a single portion and 5 rounds later took the other two, they would have 3hp/rnd regeneration for only a further 5 rounds.
The regeneration is limited to returning the user to the state they were in when the first draught was taken, but otherwise is fully effective. So, for example, if a one-armed man drunk the potion they would not re-grow the arm, but if a person lost an arm while under the influence of the potion it would regrow (in 1 round). Similarly, if a character is reduced from 72hp to 10 before taking the potion, it will only regenerate further damage suffered; it is not a healing potion.
Unlike troll regeneration, the potion will heal any damage even from fire and acid.
The potion will not protect against poison (unless it does hit point damage), nor against effects such as disintegration. A severed head will grow back from the neck down (if the head is destroyed, say by being put into a Green Devil's mouth which is actually a sphere of annihilation, then the potion will fail) in 3d6 rounds. Other than these limitations, any amount of damage which does not reduce the drinker to ash or similar will be regenerated, time permitting.
Other healing magic will work in conjunction with the potion of regeneration, but healing potions use at the same time will require a roll on the potion miscibility table (DMG p119) and any damage done as a result of that roll will not regenerate.
The active ingredient is troll-blood (which is generally very hard to obtain).
XP value: 500
GP value: 1200