How do I know if a key is pressed?

To find how long it is pressed for you would set a variable to tick() in your KeyDown function like so:

mouse=  --(However you're getting the mouse)

mouse.KeyDown:connect(function(key)
    --Do whatever you want with that key here
    pasttime=tick()
end)

Then in your KeyUp function would set another variable to tick() and subtract the pasttime variable from the variable you just set. Example:

mouse.KeyDown:connect(function(key)
    --Do whatever you want with that key here
    currenttime=tick()
    keytime=currenttime-pasttime
end)

The keytime variable would be the time the key is pressed.