Routine Plateau

[BulkCity]

My current routine is..
Monday: Shoulders and triceps
Tuesday: Back
Wednesday: rest
Thursday: Chest and bi's
Friday: legs

I usually split my workouts into two 20min sessions trying to beat the amount of sets i can do with the same weight or heavier
ex. Monday: Shoulders
Perform 10 sets in 20 minutes week 1, The following week when im training shoulders i would try perform 12 sets in 20 minutes with the same weight or heavier, trying to beat my previous record

On back day i spend the first half of my workout doing deadlifts, then the second part a 20 minute interval of trying to beat my record amount of sets from the week before. I do the same on leg day but instead of doing deadlifts i do squats.

I work a physical job so 40 minute workouts are ideal for me and have actually made great gains from doing this routine
My question is though there will be a point where i wont be able to outnumber the sets i can do in 20 minutes, pretty much reaching a plateau
So should i extend the time of the workouts and start new rocords?
or focus on making the weight heavier while the number of sets in 20 minutes stay the same?

Appreciate feedback

Stats:
Age 18
Weight 72kg
Height 172cm
Bf 12 - 14%
Goals to gain muscle mass

[hex]

ya i would suggest when you hit 15 or 13 or whatever number you choose add weight 5-20 pounds depending on the muscle and your strength go from there