8 Weeks, 4x per week
A 4-day upper/lower split on gym machines, for beginners.
Two upper-body days and two leg days over eight weeks: Upper body on Monday and Thursday, Legs on Tuesday and Friday. Every muscle group is trained twice a week, with a rest day midweek and the weekend off.
Progress: pick a weight you can lift for the bottom of the rep range with good form. When you reach the top of the range on every set, add a little weight next time.