Here is the golden rule. You should be lifting enough so that the last repetition of your set feels like a struggle to perform (without losing proper technique). Pretty simple.

Here’s the better question: how many reps should I be doing?

The answer to this question is dependent upon your strength training goals. Generally speaking if you’re looking to increase muscle strength you’re looking at lower reps (and higher weights). If you’re more so interested in muscular endurance then traditionally we look at higher reps (and lower weights). This is a very broad brush answer, but generally speaking this is the guideline.

So how does circuit or interval training fit into this?

Any timed intervals are generally going to be in the muscular endurance goal category. Higher reps & lower weights. You want to make sure you’re using a heavy enough weight that you struggle to complete the interval without losing form & technique. This may involve starting with a higher weight and dropping to a lower weight when you fatigue to complete the set. This enables you to accomplish a greater volume. The caution here is to ensure you’re not pushing too hard and putting your connective tissue & joints at risk of over-use injury. It is essential that you work up to this. Start with training at lower weights and master the form first, then layer on training volume with weights.
As with all strength work – quality always wins over quantity. Keep your pace slow & controlled (count 2-3 up & down) and keep an eye on your technique. Not sure you’re doing something correctly? Ask a qualified professional for their opinion. Look for someone who is a registered or certified Personal Trainer. Just because someone “looks” the part – doesn’t necessarily mean they know what they’re talking about. Exercise science is always changing. You want a professional guiding your course.