After endovenous laser ablation and sclerotherapy for veins, what mmHg level is usually recommended for compression stockings?

I agree with the other physicians, 20-30mm Hg is a very typical compression level to recommend after a vein procedure.  I'm sure your doctor will tell you, but that is the compression level I always suggest my patients use.

Vein specialists recommend the use of medical grade compression stockings (20-30 mmHg) for one to up to three weeks after Radiofrequency Ablation (RFA), and only one week post Sclerotherapy. It is important to follow with your provider as well as coming back for your follow up appointments after your treatments. Close communication is recommended for any question or concern. All the best!

20-25 mm Hg during 7 days. Some patients wear them for longer, for comfort. 
Wearing compression stockings prevents deep clots, reduce risks of hyper-pigmentation and gives a better outcome.

I recommend 20-30mmHG compression for 48 hours post sclerotherpay or an ablation procedure.  If you are going to wear daily compression then I would recommen the same pressure with knee length support hose.

My recommendation is to try wearing 20-30mmHg compression stockings for 7-10 days after the procedures. You should wear it during the day and can be removed to sleep. I some patient with very large veins I try to push it for 2 weeks.