Thomas E, Bianco A, Paoli A, Palma A. The Relation Between Stretching Typology and Stretching Duration: The Effects on Range of Motion. Int J Sports Med. 2018 Apr;39(4):243-254. doi: 10.1055/s-0044-101146. Epub 2018 Mar 5. PMID: 29506306.
This is where the gains drop off
No more (statistical) gains after 5 times
Hold for 30s, static, total ~5min per week (per muscle group)
It all works, static works best