There are three types of people in the gym; those who try to rest as little as possible between sets, those who spend 8 minutes chatting between every set with their buddies and those who are nerdy enough to have worked out exactly how much rest they are supposed to take. Whilst group three are probably the least fun at parties ... they are at least training correctly.
I think that the main problem with the whole ‘how long should you rest’ debate is people's perception of what intensity means. HIIT or High Intensity Interval Training is probably most responsible for this; the idea that rest periods should be very short to keep your intensity high makes it seem obvious that being intense means small resting periods.
But this isn't always the case. For example if you performed 3 sets of 6 reps on the bench press with a 60kg weight would you be more intense than someone performing 3 sets of 6 reps using 90kg? Obviously not, the other person is performing the same amount of sets and reps as you but with a heavier weight. Would the fact that they were resting longer between sets make their workout less intense? I'd argue that no, it doesn't.
Identifying Your Goals
There are 4 main goals that people have when they start training, some people only have one whilst others might have 3 or even all 4. These goals are: Weight loss, Increased Strength, Increased Hypertrophy and Increased Endurance. Deciding what goal you have in mind can change the amount of rest time that you should be aiming for.
For example, if you were looking to break the world record for a barbell squat do you think that 45 seconds rest would be effective? Clearly not. Nor would ten minute rest periods be an effective way to train for fat-loss. So deciding what it is that you want to achieve as a goal should be your first step.
This is the number one most common goal for people entering a gym; it is also quite a vague goal. If you are training for strength you would follow a program that used progressively heavier weights, if you were training for hypertrophy ... same deal. But training for fat loss can take many forms.
The best way to train for fat loss is to burn the most amount of calories during a session, whilst creating a calorie deficit through diet. Burning more calories can be managed by fitting in more exercises into a 60 minute session. One way to manage this is to shorten your rest periods to 45-60 seconds. That means more exercises completed, more calories burned.
Your workout quality will go down, you'll struggle to lift weights that are as heavy and you probably won't see any changes in muscle size or strength, but you will see improved fat loss results. You can also add in super-sets which cut down your rest periods even more.
It has been clear for quite some time that to increase your strength you require more rest time between sets. This will allow you to recover more between sets and therefore lift heavier weights. Aiming for long rest periods of around 3 minutes will suit strength training, I'd recommend timing these as it is very hard to estimate 180 seconds accurately.
A study by Schoenfeld et al (2015) found that 3 minute rests were much more effective at increasing 1 rep max scores (Bench Press and Back Squat) than 1 minute rests .
The received wisdom was that hypertrophy required shorter rest periods than strength training as shorter rest time led to increased metabolic stress. However that same study  found that the longer rest periods also suited hypertrophy as it allowed greater volume.
Volume is a measure of weight used multiplied by total reps. So 10 sets of 10 reps at 60kg would equal 6000kg volume compared to 3 sets of 5 reps at 70kg which equals 1,050kg (these sets, reps, and weights are just easy examples).
Now one thing that is important is that the number of sets and reps would have to be the same for both short and long rests for the participant to see better hypertrophy results. Meaning that the long-rest session would end up being longer.
If you are one of the few people out there who is looking to improve their muscular endurance then a 2009 study by Freitas et al has the answer for you. Apparently rest periods that last between 20 and 60 seconds is the optimal rest period for you .
In my opinion, the most effective training technique is strength or hypertrophy training combined with a calorie controlled diet. So timing your rest periods to around 3 minutes is ideal. If you are struggling for time then 2 minutes would also work, particularly for some of the less intense exercises (Lat Pulldown, Single Arm Rows, Bicep Curls etc).
But one thing that everyone should be doing is paying attention to how long they are actually resting. It is very easy to wildly over or underestimate how long you are spending looking at your phone/talking to friends/spying on other gym goers.
Matt Smith is an experienced personal trainer based in Harrow, North West London, and, as well as producing awesome content for the everyday gym goer to delve into, provides fantastic online coaching programmes that consistently get great results.
Check out more here - www.personaltrainingbymatt.com
 Schoenfeld, B., Pope, Z., Benik, F., Hester, G., Sellers, J., Nooner, J., Just, B. 2015. Longer inter-set rest periods enhance muscle strength and hypertrophy in resistance-trained men. The Journal of Strength & Conditioning Research 30(7): 1805-12
 Freitas de Salles, B., Simão, R., Miranda, F., da Silva Novaes, J., Lemos, A., Willardson, J. 2009. Rest intervals between sets in strength training. Sports Medicine 39(9): 765-77