If it helps, I do about 1 hour 3x per week. You might be able to get away with 1 hour 2x per week and still make some gains--albeit a bit more slowly. The key to being efficient with your time is to perform exercises that recruit as many muscles at the same time time as possible. That also means no machines. We're talking free weights here.
A beginner's weight training program will consist of 5 exercises, split between two workout plans, with only three exercises per session. Common rep schemes are either 5 sets of 5 reps or 3 sets of 5 reps.
The top four exercises you will see are:
- Squats (the foundation of barbell strength training)
- Overhead press
- Bench press
and the remaining exercise will likely be (depending on what program you start with) either:
- Barbell rows
- or Power cleans
The two beginners programs that seem to be really good are StrongLifts 5x5 (SL) and Starting Strength (SS). Medhi (the guy behind SL) has been going on a salesmanship spree lately, so you will have a lot to ignore, but the core program he has works and works well. His is a 5 sets/5 reps scheme. Rippetoe (the guy behind SS) is a pretty down to earth kind of guy with a dry sense of humor. His book is very good as far as teaching technique and what to look for as a beginner. SS is a 3 sets/5 reps scheme.
So, you should be very effective at building strength using:
- Free weight full body compound exercises (such as what was mentioned above)
- 3 days, about 1 hour per day--and a full day of rest in between (very important)
- 3x5 minimum, 5x5 maximum load
- Progressive loading (adding 5 lbs each session on all the exercises)
I personally did the SL route for the past 11 weeks, but will be switching to SS after week 12. The primary reasons are that 5x5 is getting a bit heavy now and I'll appreciate the lighter load, and power cleans are fun and want to incorporate them into my workout.