Knee Ability Flexibility Standards
The following list of videos shows me demonstrating each Flexibility "Standard," - a mark I've found puts you in the upper 1% of the population for flexibility - while giving you the ATG strength exercises that assist in achieving - and in some cases fully maintaining - the Flexibility Standard.
I don't think you have to be worried about stretching "too much," but I've seen best results by working on each stretch 3 times per week, until the Standard is achieved, at which point you can reduce frequency. In some cases, I no longer have to do the stretch at all, because specific ATG exercises allow me to hold the Standard without any direct work on the stretch.
Please bear in mind this is not an "end-all/be-all" flexibility regimen. Rather, this is a list of measurable targets which anyone can REALISTICALLY achieve, yet which put you in the upper 1% of flexibility. For someone with more extreme flexibility goals, this list of stretches and correlating strength exercises could create a wonderful base for you...
1. Quad/Hip-Flexor Flexibility Standard: Couch Stretch with Knee and Shoulders to Wall and no strain. Assisted and eventually fully maintained by ATG Split Squat + KOT Squat!

2. Calf Flexibility is the only stretch without measurable Standard. The "Standard" is a subjective one: no longer feeling a calf stretch while standing on slantboard. Assisted by FHL Calf Raise and Single-Leg Standing Calf Raise.

3. Hamstring Flexibility Standard: Palms to Floor Pike with Full Quad Contraction. Assisted by and eventually fully maintained by Jefferson Curl (slantboard trick can help, too!)

4. Piriformis Flexibility Standard: Floor Stretch with Front Leg 90 Degrees, Back Leg Straight, and Head to Ankle.

5. Pancake Flexibility Standard: Head to Floor without strain. Assisted by and eventually fully maintained by Seated Goodmorning + ATG-style RDL!

6. Groin Flexibility Standard: Knees to Floor. (Can add weight like this, too!)

Yours in Bulletproofing, Ben