I do resistance training every other day for 20 to 25 minutes a day. (Or would be if I hadn't developed an upper respiratory infection the day I started writing this thread! ><)
Upper body Monday/Wednesday, Legs Friday. Maybe some off work on Sunday.
Doing that I get along just fine. A day is enough to recover from a workout unless you tore yourself to pieces in the previous workout. (I find it's very easy for me to get in a leg workout that is intense enough I feel it for three days.)
D. O. N. T. S. K. I. P. L. E. G. D. A. Y.
Once a week, even for an hour, wouldn't be enough for me. That's why I don't just advocate resistance training, I advocate high intensity resistance training. Because you can fit it in a lot of places you can't reasonably fit an hour long workout. Working out shoudn't take a huge chunk of your week. A quick, intense circuit is something you can fit in right when you get home, do it for 30 minutes, get your shower in, get to bed. (Although working out before bed has been reported to create some insomnia issues due to over-stimulation of the nervous system.)
As for what is "enough" for you, the question I return with "What are your goals?" For some people that are just like "I want to be more active", sure, a day a week is enough. If you want to lose weight through physical activity, build muscle or build strength, then no, I would say one day per week isn't enough if you want to see any results (and get motivated by them) sooner than in a few months. That's what really drove me to up my routine to at least 3 days a week. When I started I was doing an intense 30 minute workout a week for a few weeks and it just wasn't causing that much change. And I spent the other half of the week full of energy and feeling like I should do more.
Be careful about trying to do too much in one day too. If you end up really taxing both your upper and lower body in one workout, well, guess what? The next day you may be functionally crippled as every major muscle group hurts and is tight. You also tend to recruit a lot of different muscles in workouts, which if every major muscle group has been taxed during a workout, you may find finishing your workout is harder toward the end because you've tapped out everything.
The only muscle group I advocate working every single day if you have time is your core. It's built for that kind of stress.
I'm also very in to kettlebell swings right now. Unfortunately I feel like I just can't get the right activation for it. I've watched videos, I've done the research, I practice, I just haven't formed that mind/muscle connection to my ass and hips, so I find the kettle bell swing motion strange and like I'm still doing it wrong. I'm not pulling with my shoulders yet my shoulders are what end up feeling tired. I usually do about 40 in my regular workout routine. Could probably do more but I have to leave energy for squats and lunges, too.
Also planning on trying the Turkish Get Up here soon. Only thing that has stopped me is how technical the exercise looks.
I'm sticking mostly to the kettle bell swings, I really want to build explosiveness and full body power more than muscle anyway.
Do goblet squats with your kettle bell too. Might as well right? You got the necessary equipment and a squat is both a big compound exercise and a natural compliment to the areas worked by the kettlebell swings. Squats will give you plenty of explosive power from the crouched position, and a stronger base of support in a (typically weak for most people) stressed position.
Basically, to everyone doing resistance training: your body adapts. It gets used to, rather quickly, the things it's asked to do. The same weight, the same rep range, the same exercise, your body and you eventually get comfortable with them. And it doesn't have to work as hard to do them. If it's not working hard, it's not being challenged. If it's not being challenged, you're burning fewer calories and building less muscle.
So if you have a favorite exercise, consider alternating to your LEAST favorite exercise. It's probably your least favorite because it makes you feel the most uncomfortable and makes you feel the weakest. That's the exercise you should attack. Not the one you've become "good at." That's not to say don't do your favorite exercises. What I'm saying is: keep your body guessing. Change what's being asked of it regularly with variations in the type of exercise, the weight range, the rep range and particular form it's done in. Part o dat "Muscle confusion" everyone likes talking about.
That's currently one of my big problems, actually, is that I don't have the weights to do solid progression and my body is adapting to the weight ranges I'm throwing at it. Not much to be done there short of buying more weight / getting access to more weights or increasing reps and picking harder forms. Kind of sucks. I feel like I've been stuck in the 30 pound range for dumbbells my whole life. I was doing this back in high school as my near max. I feel like I'm stuck in that no man's land where I'm doing slightly higher than "average lady weight" and the beginnings of actual "man weight." I watch fitness videos of chicks doing 40 pnd dumbbell whatever and look at myself and feelsbadman.