How much weight may I lose after facelift without consequences?

Great Question! 
My answer is LOSE AS MUCH WEIGHT AS YOU CAN to reach your safe weight (your primary doctor can help you determine that). The facial drooping (if present) is the last thing for you to think of! The benefits of weight loss in health, quality of life, etc completely trump concerns of facial drooping. 
Even with facelift or another tightening procedure, losing the weight will still be way cheaper than keeping the weight on (increased medical bills, medications, etc)
I'm proud of you for entering the weight loss fight! Day by day you'll reach your goal!
Hope this helps and God Bless!

Dr. Robb

Thank you very much for your
question, as it is a good one. Small weight fluctuations will have little
consequence on the facelift results. If you lose more than 10-15 pounds, subtle
changes may be seen. A massive weight loss, however, may mean more visible
results; however this will depend on your skin elasticity and individual
situation.  If you have concerns about
the changes to your facelift results, I suggest consulting with a board
certified plastic surgeon. He/she will answer your questions and discuss your
treatment options.

Fullness in your face can be a hallmark of youth which is why we so often place fat grafts in faces to enhance fullness.  Be aware that too much weight loss is likely to decrease facial fat volume which can make you look older.  If you plan to lose a lot of weight you should do so before undergoing a facelift if  possible.