When restarting Propecia, how long before it becomes effective, and to what extent?

It is best to discuss medication issues with the doctor that knows you and has prescribed that medication to you.  In general, you cannot grow the hair back that was lost after stopping Propecia.  This is called "catch up" hair loss.

It can take 3-6 months to see improvement. Usually a combination approach is best. I suggest seeing a hair loss expert to evaluate and go over your condition and provide you information about your best treatment options. There are great non-invasive options like PRP/progesterone and/or hair transplantation. See an expert. Best, Dr. Emer

I
think of finasteride as life support for the hairs that are miniaturized and
are on the border of being lost.  Once
you stopped the finasteride and your hair started falling out again, those
hairs are lost for good.  Thankfully you
restarted the finasteride and will see hair stabilization with your remaining
hair within 6 months.  It is best to be
seen by a hair transplant specialist to review your hair loss history/family
history and perform a microscopic exam. 
Only then can your doctor come up with a master plan and discuss this
with you with some sense of accuracy.

I rarely find that patients who stop finasteride actually get back to the same level after restarting again. But in general, after about 9-12 months you'll have a good sense of what improvement you'll achieve. 

Stopping and restarting finasteride does not guarantee that the hairs that were lost for the 2.5 months while you were off of it, will return. Time will tell you that answer