Great question actually. There is not an answer that is gospel however, I have found after my boa goes blue, its about 5 days until the shed. The actual FULL process takes quite a bit of time and sometimes we don't realize the changes immediately. The blue phase is in the middle of it all.
You will notice the snake will clear up a day or 2 prior to the shed and then, boom!! New bright colors!
Keep a log of when you notice the blue eyes and then when the shed happens. Have fun!