The answer to that would seem obvious however, if you take in to consideration age, body weight, purpose of training, sessions per week then your limit will vary according to which combination fits your physical make up, fitness level, and available time. And also assuming only changes to diet will be made, and supplements are restricted to vitamins and minerals.
Weight training limits usually begin with establishing a one rep max. This approach does make much sense as it doesn’t take in to consideration an individual fitness level and or endurance. It only demonstrates the degree of strength for 1 rep and so therefore it’s meaningless when it comes to setting a basis for weight training and or using weights to train for fitness as opposed to just strength. You’re not going to do a whole workout using a weight that is too heavy to do more than 1 rep.
It would be more accurate to set a limit by using a combination that includes both strength and fitness level and so instead of 1 rep max use a rep level of 8 rep max.
Begin with selecting a dumbbell of say 10kg. Do an 8 rep bicep curl and see how it goes. If it went without a problem then rest for 3 minutes and select a 12kg and repeat the process adding weight until you can’t quite achieve 8 reps and ensuring the technique is 100% correct right through to the last rep
From this you’ve established your maximum dumbbell weight for a one arm bicep curl. Now run through all the exercises in your program using the same method. Correct technique is essential otherwise the time is wasted and you’ll have to start again.
What you’ve now done is set up a base for your weight training exercises.
From here you can stay at that level or increase the weights and reps as strength and endurance permits. Increasing the reps will increase endurance faster than increasing the weights too quickly. When you have reached a good level of endurance (fitness) drop back to 8 reps and add more weight. Repeat the process until you feel you’ve reached your peak combination weight and endurance level.
From there drop back to your original weight and reps for a 2 week period allowing for an overall rest. Build up the weight again gradually making absolutely sure your technique can’t be faulted, so by the time you are back to where you last reached a peak, your strength will start to increase past that peak.
The ‘rest’ period is essentially a time to really make certain the technique is right because without that you can only go so far.
If it gets boring get on the bike, rowing machine, treadmill or cross trainer and hammer those in between weight sessions, but stick with the rest/technique method for long term results.