Per the Player’s Handbook, proficiency with an herbalism kit can be used to gain your proficiency bonus on ability checks made to identify and apply herbs. In addition, the kit is considered a tool, unlike the healer’s kit which is adventuring gear. Proficiency with this tool is required in order to craft antitoxin and potions of healing, though no specific rules are given for doing so. As the kit is a tool, we can surmise that the downtime crafting rules in the PHB can apply.
Both antitoxins and potions of healing have a listed value of 50gp. Thus, we can extrapolate that crafting one of either of these will take 10 days of downtime (50gp / 5 per day of work). The rare herbs and other materials required to refine them, brew them, and hold the resulting potion would cost 25gp per unit crafted. Each additional herbalist working on the project can add an additional 5gp of progress per day, so that two herbalists working together can cut the time to 5 days, while 3 reduces it further to 4 days (50 / 15 = 3.333 rounded up).
As a house rule, a character proficient with an herbalism kit may make one Intelligence or Wisdom check (player’s choice) including their proficiency bonus for each day of overland travel through the wilderness or underdark in order to locate and collect rare herbs useable in the crafting of antitoxins or healing potions (again player’s choice). The base DC of this check is 10 and results in 5gp of herbs weighing 1/10th of a pound found and collected (the player should track their herb value on their character sheet). For each point the roll exceeds 10, an additional 5gp of herbs is found.
The cost to craft a potion or antitoxin can be reduced by herbs gathered by the character or an ally. For each gold piece value of herbs expended, reduce the cost of creating the potion by 1 gp, to a minimum of 1 gp (the cost of a vial to store the completed product).
Crafting while adventuring
During a long rest, a character proficient with herbalism and possessing an herbalism kit can make a single Intelligence or Wisdom check with proficiency at DC 15 in order to make progress on a potion or antitoxin as if they were using a day of downtime. Success adds 1 day/5gp toward the creation of one item.
Potions of healing
The potions of healing created by an herbalist are identical to a magical potions of healing listed in the Dungeon Masters Guide, but do not follow the rules for magical item creation. An herbalist can only create the base potion of healing (2d4+2 restored hit points) using this crafting method. Stronger healing potions (superior, etc) can be crafted using the magical item crafting rules in the DMG.
Antitoxin cannot remove poison damage already sustained from the damage of a poison attack originating before the consumption of the antitoxin. It can, when consumed, allow a second saving throw (with advantage) against an ongoing effect causing the poisoned condition. If this save is successful, both the poisoned condition, and any other side-effects of the poison, such as ongoing damage or the paralyzed or immobilized condition are also removed.