NOTE: Thanks to janiwesterling for pointing out that they've changed the times for everything in the latest update (shortest pirouette is 10mins for example) so these calculations are no longer accurate.
First, you'll need to get a pre-teen sim. This will involve being married, buying a crib and you will need to bake 2 birthday cakes - each 1 takes a day to bake. You'll start with a baby, spending 1 cake to grow into a toddler, then the 2nd cake to become a pre-teen. Also, you'll need to build the community/rec center in the upper left corner. This is where the sim needs to go to practice.
The easiest way to do this is to have your pre-teen sim keep stretching. I started by clicking on the record player and having my sim do pirouettes. The XP went up very slowly, and these events took a minimum of 4 minutes.
The stretching exercise is 30 seconds long, and gains a specific % depending on your level:
1 - 25% - 4 times (2 minutes) - Thanks to AP E JC for this info
2 - 12% - 9 times (4.5 minutes)
3 - 6% - 17 times (8.5 minutes)
4 - 3% - 34 times (17 minutes)
5 - 1% - 100 times (50 minutes) (TBH, this seemed to go faster for me, but I didn't count)
So, in total you would need just over 82 minutes to do this.