it all depends
- I move to https one quality site most keywords from 1-10 in google with amazing traffic
and gap was only 3 days.
- i move also plenty of no-quality sites or where rankings was around ~30 and gap was even a 3-4 weeks.
if you do for all non-www and www redirection to https and when you add all types to google webmaster tools you don't should get any drops of ranking - your site will be re-indexed and all links also will be re-indexed from no http to https and it will get a time.
If you have a chance good step will be to change some link to your site from http to https
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