When should I consider surgical intervention for Achilles tendonitis?

Try non-op 1st. See your ors and discuss this w/ him and get examined etc. In general, surgical intervention for achilles " tendinitis " is not very common.The majority of cases can be treated non- operatively.Stretching, inserts, nsaids , activity modification . Occasionally , injections of prp( from your own blood, not steroid) might help. Mri can help assess the amount of tendon damage.
Chronic pain. If you have tried, injections, orthotics, physical therapy then you may want to consider surgery.