How long does it take to build muscle?

There's no straight forward answer without knowing your full circumstances but here's my quick response for what it's worth. If it's mass you're after then stick to around 3 sets of 8 to 10 reps, if it's general tone and strength then 12 to 15 reps over 3 sets of a much lighter weight. No more than an hour as after that the return on investment drops off quite steeply. You won't build muscle without nutrition or rest so think about protein and sleep. Also if you're aiming to train to 6 times a week you'll need to vary your workout so that you don't train the same muscle twice within around 3 days. If this is your first gym session for awhile then keep it light and build up over 8 weeks or so. Anything other will risk injury. 6 sessions for the unaccustomed sounds a bit too much if I am honest.

If I have a lay off from the gym for a month or so then I can expect to put on 2lb within the first fortnight of returning but then it evens out. I don't train to bulk up these days and focus more on the athletic, just a pity I haven't been consistent for ooooh errr. back on the wagon now though!

It depends what kind of strength training you're doing. If you're doing full body workouts, Give yourself at least a day in between workouts to let your muscles recover. If you are splitting your workouts into upper and lower body workouts, you can workout more, but give yourself at least two days a week to rest and recover. Some light cardio on your rest days is okay. Make sure to consume at least 20 grams of protein within half an hour of your workout. I like to alternate my workouts between heavy weights/low reps and light weights/higher reps. If you are working out for 30 minutes at a time, I would probably go with the upper body and lower body workouts, as opposed to a full body workout. That way you can work out on consecuetive days without overworking your muscles. Do compound or multijoint exercises like squats, deadlifts, bench presses and rows before doing your single joint exercises like dumbbell curls or leg extensions. It's impossible to say how much muscle you'll gain because everybody's body is different, but you should start seeing results after a couple of weeks, whether it's gains in strength or your physical appearance. Take a complete set of body measurements before you start, so you can compare after a couple of weeks and see the improvements. Don't be discouraged if your weight loss slows down or stops completely for the first couple of weeks. When you start a new or different exercise routine, your muscles tend to hold more water to help repair themselves. It's just temporary until your muscles adapt to your new routine! Hope this helps. Good luck!