First off, I second anything Ron Killian has stated, so I won't necessarily repeat what has already been said, however, I would also like to add a few points.
Your Question: What is the right length to set up autoresponder for?
Email series should be as long as you need them to be to get the subject matter point across. If it can be done in 3 messages, then, 3 it is. If it needs to be spanned across 150 emails, then 150 it is (although you might want to consider turning THAT one into a PAID product, LOL).
Your Question: What do you do when the autoresponder is finished?
It is NEVER finished. Your job as a marketer is to continuously market to your subscribers. Continue emailing them every other day or so with relevant, valuable information and be sure to throw in an offer or two every 2 or 3 emails you send out. There is no such thing as a niche with limited information. Everyday something new can be learned. So, even when you're done telling your subscribers everything you know, find out something new and pass it along. THAT is why they are subscribed to your list and that is why they will stay.
I also wanted to touch on something Ron mentioned above... "List hygiene"
List Hygiene is extremely important now a days. For starters, yes, most email marketing companies now charge for the number of subscribers you have, even if they have unsubbed, they are still considered an "assett" if they are taking up space in the database, so, you definitely wanna get rid of as many Non Action Takers as possible, however, there is a proper way to do so.
For this post, I will refer to Non Action Takers as NAT's (nothing personal of offensive meant by that, but that's just what the initials spell out, lol)...
Anyways, every 3 or 4 months, I will usually gather up a list of NATs and assign them to a "group" that I can send email to separately. I will send out a series of 3 emails to them explaining that since they do not open my emails, I can only assume they are no longer interested and do not wish to waste either of our time.
The 1st email is a simple nudge letting them know I notice they do not open my emails. I also invite them to contact me and let me know if there is anything I can do to gain their interest back... This email is also a further test since I can now see who, on that list, opens (or doesn't open) THAT email, lol.
The 2nd email lets them know that I will send them one final email and that if they are indeed interested in staying on the list, they should be sure to open the 3rd email which will be sent out in X number of days. (I usually do the next day or 2 days max since I am not fresh in their mind).
The 3rd, and final email, is a Good Bye message along with a link to the subscribe page. I send the link because I delete the users as soon as I see the message has been sent to all those within that segmented list...
Also, since my mailing list providers include link tracking, every email to these segmented subscribers (NATs) includes a link they can click on if they wish to remain subscribed. I check the link stats to see who clicked on the link. I then ( A ) remove them from the segmented list, ( B ) add them to a 2nd segmented list which I use to send out a "Thank You For Remaining A Subscriber" email and then ( C ) i finally delete them from that segmented list so they do not continue to receive the 2nd or 3rd email in this set.
Now, this is how "I" clean my list of NAT's. I am not saying this is the ONLY way or that this is the way YOU should do it, just giving an example of how "I" clean my lists...
If I have confused you, or you would like some personal assistance, I'd be glad to lend a hand any way I can.