How would I make a weapon hitbox that does constant damage over time?

You already have half the code, just use your table in an loop Something like:

local DebounceTable = {}

local StartTime = tick()
local HurtTimeTick = 0.0 -- change this to how quick or slow you want it to hurt them
while true do
    local NowTime = tick()
    local DTime = NowTime - StartTime
    if(DTime >= HurtTimeTick) then
        StartTime = tick()
        for Player,_ in pairs(DebounceTable) do
            if(Player ~= nil) then
                Player.Humanoid:TakeDamage(10)
            end
        end
        --print(DTime)
    end
    wait()
end

This is a bit of a hack. But it's a way that would keep track of players/npcs that have contacted the hit box. then your timer in the touch event would remove them after a 0.5 sec delay.

The code in full:

local DebounceTable = {}

script.Parent.Parent:WaitForChild("HITBOX").Touched:Connect(function(objectThatTouchesTheHitbox) 
    local PossibleHuman = objectThatTouchesTheHitbox.Parent
    if objectThatTouchesTheHitbox.Parent then
        if PossibleHuman:FindFirstChild("Humanoid") then
            if DebounceTable[PossibleHuman] == true then return end
            DebounceTable[PossibleHuman] = true
            --PossibleHuman.Humanoid:TakeDamage(10)
            wait(0.5) -- change this to change the duration of pain for the humanoid
            DebounceTable[PossibleHuman] = nil
        end
    end
end)

-- Coment out this bit if you dont want the Touch ended  to fire when the npc/player moves away
script.Parent.Parent:WaitForChild("HITBOX").Touched:Connect(function(objectThatTouchesTheHitbox) 
    local PossibleHuman = objectThatTouchesTheHitbox.Parent
    print(tostring(PossibleHuman).."untouched me!")
    if objectThatTouchesTheHitbox.Parent and  DebounceTable[PossibleHuman] ~= nil then
        DebounceTable[PossibleHuman] = nil
    end
end)

local StartTime = tick() -- start a timer and record our start time

local HurtTimeTick = 0.0 -- change this to speed up the Taken damage the smaller, the faster it will take health away, larger number means slower!

while true do -- main "Pain loop"
    local NowTime = tick()
    local DTime = NowTime - StartTime
    if(DTime >= HurtTimeTick) then
        StartTime = tick()
        for Player,_ in pairs(DebounceTable) do
            if(Player ~= nil) then
                Player.Humanoid:TakeDamage(10)
            end
        end
        --print(DTime)
    end
    wait()
end

Hope this helps! :)

Rather than using a hitbox, you could use Magnitude.

local Tool = script.Parent
local Handle = Tool:WaitForChild('Handle')

local Players = game:GetService('Players')

local Dmg = 10 -- damage per rate
local dmgRate = 1 -- how often it damages a player in seconds
local minDistance = 30 -- how close someone has to be in order to be damaged

Tool.Equipped:Connect(function()
    local Player = Players:GetPlayerFromCharacter(Tool.Parent)
    local Character = Player.Character
    if Character then
        local humanoidRootPart = Character:FindFirstChild('HumanoidRootPart')
        if humanoidRootPart then
            while true do
                task.wait(dmgRate)
                for _,player in pairs(Players:GetPlayers()) do
                    if player ~= Player then
                        local pCharacter = player.Character
                        if pCharacter then
                            local pHumanoid, pHumanoidRootPart = pCharacter:FindFirstChild('Humanoid'), pCharacter:FindFirstChild('HumanoidRootPart')
                        if pHumanoid and pHumanoidRootPart then
                            if (pHumanoidRootPart.Position - humanoidRootPart.Position).Magnitude <= minDistance then
                                pHumanoid:TakeDamage(Dmg)
                            end
                        end
                    end
                end
                if Tool.Parent ~= Character then
                    break
                end
            end
        end
    end
end)