[Solved myself] How can I detect if player is holding left mouse button?

By whatever method you are using to detect user inputs, you can have a variable that stores whether the mouse is down as a boolean value. Then, have a function that sets it to true when it detects the beginning of the click, and false when the end of the click is detected.

If you want this, then you need a script that detects input on left click that began and ended. You can see how long player has held the left click with a timer.

Use this:

local UIS = game:GetService("UserInputService")

local held = false
local timer = 0

UIS.InputBegan:Connect(function(key, gameproccessed)
if key.UserInputType == Enum.UserInputType.MouseButton1 and not gameproccessed then
held = true
end
end)

UIS.InputEnd:Connect(function(key, gameproccessed)
if key.UserInputType == Enum.UserInputType.MouseButton1 then
held = false
end
end)

while true do
wait()

if held then
timer = timer + 0.03
else
timer = 0
end

if timer >= 0.7 then -- you can modify the timer to whatever you like
--code here
end
end

Found a solution. This can be done by finding player's mouse and repeating fire() function. script would look like this:

function fire()
--code here
end

mouse.Button1Down:connect(function()
    hold = true
end)
mouse.Button1Up:connect(function()
    hold = false
end)

tool.Equipped:connect(function()
    tool.Activated:connect(function()
    repeat
    fire()
    wait()
    until hold == false
end)
end)