How Long After Invisalign Do I Need a Retainer?

Hello there,You would have to wear the retainer indefinitely.  Another option would be having a permanent piece of wire placed behind your teeth that would keep your teeth aligned and prevent the need for a retainer.  That area would be more likely to gather food and subsequently bacteria growth, so it would require extra special care to keep clean. 

First, you are a very pretty women just as you are.  If you do want to improve your smile I suggest instead of moving teeth you consider orthopedically growing your jaw using the DNA Appliance and Epigenetic Orthodontics. The DNA is only worn 12-14 hours a day and does not need lifetime retention.It will also help the dark areas under your eyes.It will create a wider fuller smile as well.

The retention phase of treatment comes after tooth
movements using the aligners have been completed.
Due to the tendency of teeth to shift in the human
dentition, you can expect that your teeth will
naturally begin to shift back to their original position
once your prescribed course of clear aligner treatment
is complete. For this reason, retention is a lifelong
commitment and you MUST follow the retention
instructions prescribed by your doctor. some instances call for a lingual bar that is bonded to the back of your front teeth. These are rigid metal
reinforcements placed behind your teeth to prevent
them from moving. In addition, you will need to wear retainers
indefinitely until your doctor tells you that you no
longer need them. Retainers are typically worn full time,
at least 22 hours a day, for the first few weeks to a few months of use.
After that initial retention time, you may, with permission
from your doctor, start to decrease the amount of
time during which you wear your retainers by
approximately two hours a day for each week.
After a few months of gradually wearing the retainers
less frequently throughout the day, you may, with
permission from your doctor, begin wearing your
retainers at night only. Please keep in mind that
decreasing the amount of time during which you
wear your retainers depends on the retainers being
comfortable and loose when placed in the mouth. If
your retainers feel tight or you feel pressure, you
must revert back to and repeat the schedule of the
previous week.
You must wear your retainers every night for the first
year of use. After the first year, you may, with
permission from your doctor, begin wearing them
every other night, provided that they are loose and
fitting well; otherwise, you must continue to wear
them every night. You should bring your retainers to all future followup
appointments so that your doctor can assess their
fit and function and make adjustments as necessary

If you would like your teeth to stay perfect after you finish Invisalign, a properly fitting retainer is essential. You don't have to wear the retainer all of the time but at least 3 to 4 nights per week. If you ever see your teeth shifting or space re-opening, then you can increase to 6-7 nights per week. Also, if you do not like wearing the retainer while you sleep, you can wear it during the day instead for around 6-8 hours.
Retainers are like orthodontic insurance. If you wear a properly fitting retainer every night, your teeth will not shift! If you don't wear a retainer, it is like throwing the cards up in the air. You don't know how they will land. For some people (the lucky ones), teeth don't shift; but for a lot of people, the teeth will shift over time. 
I still wear a retainer every night. Yes, I have to because I'm an orthodontist. But even if I wasn't an orthodontist, I like my smile and want it stay like this for the rest of my life, and retainers are the answer.

Hello,
I love that you are looking at all the commitments you need to make before you start treatment. A fun way to say it is “you only have to wear your retainer for as long as you want straight teeth”. But joking aside, teeth shift all your life. Yes, retainers are a life-time commitment. 
For the bottom teeth, sometimes there is an option of placing a fixed retainer that is glued behind your teeth, then you would not have to worry about remembering to wear a retainer. But on the top, you have to wear your retainer at nights indefinitely. 
Best,
Dr Sepi Torkan