It’s so variable but 2-4 months is the wrong time frame to looks at, try 2-4 years then if you get gains more quickly it’s a bonus. Ok, at least a year then.
If you are lucky you’ll get a quick gain at the start (maybe as much as 1/2” in a month), if you are unlucky you won’t. This initial gain might be a matter of recovering length (ie regaining length that existed but has shrunk back over the years) and at 18, you are unlikely to have any of that.
People of any age seem to gain quickly at the start, if they gain quickly at all, and this is probably a matter of ligament gains (exposing the hidden inner penis) as opposed to tunica gain (expanding the existing penis).
In 4 months, you can expect to almost have a reasonable jelqing technique down.
Have fun.