Answer:
6 Minutes at least.
Step-by-step explanation:
In 6 minutes you would have 18 and your freind would have 18.
Considering they're getting 2 more each wave, we can apply simple math to this.
6+2 and onwards,
0+3 and onwards until it surpasses the first value.
I calculated this, and it seems it takes around 6 minutes to catch up to the friend at 18 exercises.