How long does it take to get good at metal guitar?
Short answer
A few months of focused practice can produce usable, in-time riffs. Controlled speed and musical lead typically take years. There is no honest single number.
Detailed explanation
“Good” hides three different targets: (1) play a simple song section cleanly with a click, (2) rumble through intermediate riffs at original tempo, (3) improvise or compose leads that hold up on a recording. Target 1 is weeks to months if you follow a path like the beginner path. Target 3 is measured in years of deliberate work.
Hours of noodling do not convert 1:1 into skill. Quality of reps—accuracy just under breakdown tempo—matters more. See deliberate practice.