over training is extremely difficult to ever get to , 98% of us don't need to ever worry that we will over train.. 'over reaching' a bit, maybe. Olympic athletes training 8-10 hours a day, Navy Seals training 20 hours a day with little sleep and little food can get to a state of 'over training', , but your average gym goer will never get there. If your worried about the 'over reach' then just increase your food intake and sleep.
If your training 90 mins with weights in the gym , then that means your workouts are not high intensity. Its impossible to train with super high intensity and go for that long, maybe an hour tops.
so you have to decide, do you want moderate intensity with high volume (thus a longer workout) or do you want high intensity with lower volume (thus a shorter workout) . It depends on which one may work for your body type and genetics.
a simple guideline in regards to volume is to pick a number of working sets per muscle group per week. So lets say you want to shoot for 20 working sets for chest.
that could look like this
5 sets flat bench press
5 sets incline dumbbell press
5 sets flies
5 sets cable cross over
** this is just your working sets, not including any warm up sets
that may take about an hour and 20 mins to complete.. that can be your chest day that gets hit once per week, or you can break that down in half and mix it with another muscle group (like making it a 'press' day and hitting chest and triceps on the same day) and end up hitting the muscle twice a week.
once you get that nailed down you need to think about rep ranges. its best, imo, to mix it up. one set might be a heavy set and your doing 6-10 reps. One set may be a light set and your shooting for 12-15. Or you could pyramid up where you start light and keep adding weight until you get to your heavy heavy set 5-6 reps, then you start removing weight and go back down and lift to failure.
** and of course rep ranges are going to vary depending on body part as well. example, for me my arms do not grow by using heavy weight and low reps , I have to use rep ranges from 12-30 with the strictest form possible. but with chest and shoulders I can go heavy with low reps.
after that your going to have to think about the Time Under Tension. that is the amount of time the working muscle is under the load with the weight . the more time under tension you can get the more stimulation of the muscle you get.
example, if you do bench press and you do 10 reps and its one second up for the positive and one second down for the negative and your just simply moving the weight from point a to point b, for essential '20 seconds' of 'time under tension', well that is not near as much stimulation as you could achieve using the same weight, set and rep range. instead really concentrate on contracting and squeezing the muscle you are working, shoot for 4 seconds for the negative and 2 seconds for the positive motion , that results in 60 seconds of time under tension for the exact same set that would have just been 20 seconds, that is 3 TIMES THE AMOUNT OF TUT (you get way more bang for your buck when you concentrate on TUT).
so imagine how much more gains you will have over a period of one year if you have focused on Time Under Tension, your muscles will have hours and hours of more time under tension then if you don't use it,, that means more growth (without ever increasing the amount of volume or reps you are doing)
just a few things to think about