I'm a 15 and a half year old beginner and I've been lifting for about 2 months. I'm 5'8 and i weigh 143 pounds.
I'm currently doing a fullbody split 3x a week.
I'm staying in the 8-12 rep range.
How often should I be able to increase the weight on my lifts?
How much stronger should I be getting after each workout?
Should I be able to do heavier weights every week?
You don't necessarily have to use heavier weights every week but you should be progressing at least in reps.
When you get to the higher end of the 8-12 rep range you're currently working in add say, 5lbs and work your way back up to 12.
There's no way to know how much stronger you should getting from workout to workout but ideally, since you're young, you should progress pretty fast.
As to whether you strength is going up will depend a lot on the things that you do outside of the gym. Are you eating enough? Are you getting enough rest? Do you have any injuries. If you are not eating enough and not getting enough rest your strength can go down.
If you are a real newbie your strength may climb each week for a while, but dont expect this to continue forever. You will have good weeks and bad weeks.
As to when to increase the weights, if you are working in the 8-12 rep range I would suggest that when you get an exercise up to the 12 reps, increase the weight to something you can do at the 8 rep range.
Just a random question for everyone, at what age should you stop counting your age in half years?
The answers to your questions also depend on what lift you are talking about. If you are talking squats and deadlifts, then yes I would expect you could increase these every week for quite a while. If you mean triceps kickbacks, give yourself a slap. What the hell are you doing?
If you are not doing squats and deadlifts, start. Now.
How much do you eat? If it is less than 4000 cal/day, eat more. Don't go to 4000 cal in one jump, you will probably make yourself sick. Try increasing 250-300/day every week until you get 4000.
add 5#s every week
you will add 250 pounds to all of your lifts in just a year
and that even gives you two weeks to take off
i remember Staley talking about this last year in the AZ
Aim to add 5 lbs on your lifts every wokrout and on the 5th week, deload 5 lbs and then start loading up again on the 6th week