How long did it take you to get a decent amount of books on your ereader?

I have over 300 real books and I can't afford to buy them all in a Kindle edition.

Do free books change often enough to build a library?

Ive only had my kindle for a couple weeks and have about a 100 books on it. Some were freebies, some were paid for. Some were pdfs i already had on my computer.

Free books are coming out all the time, and there's the free places for stuff in the Public Domain, and web sites that collect info on original fiction posted by the authors on their web sites. Then check out the ebook stores. Harlequin at one time had about 60 free books at one time. Fictionwise and AllRomance ebook stores offer some free ones as well as Smashwords and Lulu.

And I already had a ton of fanfic. So each machine I've had in the past to read files on was pretty much as full as I wanted it to be. Lol.

It's not an either/or thing. You can keep your paper books :-) although I find I did put as many of my favorites as possible on my Kindle or Nook. Maybe I read Jane Eyre only once a year, but it's comforting to have her companionship wherever I go. 

The first year I had my Kindle (I've had it about 3 years), there weren't anywhere near the freebies that are offered now, and many of the things offered were not to my taste -- science fiction, thrillers, and erotica. I looked for the ones with the best reviews on Goodreads or similar places or looked at the 3-star reviews on Amazon and found that if I chose carefully, I was enjoying things outside my usual reading comfort zone and finding new authors. That's one of the good things about freebies, and one of the good things that it shares with PBS - giving one the courage to read something one ordinarily would not. 

Recently we've been awash in Christian fiction and romance, again not my favorite areas, but I've found that by being selective, and with the multitudes of public domain offerings and indie offerings, I've got a lovely little library of about 1300 books. About 1/2 my books are from Amazon, and the rest from other sources. 

Check out the threads for freebies, look at the links for things like Books on the Knob http://booksontheknob.blogspot.com/ or ereaderiq http://www.ereaderiq.com/free/, and be comforted. And the nice thing is, all those books, and the ereader still only weighs about half a pound. 

Edited to add links. 

I've had my ereader for just over a year.  I still read as many paper books as I ever did because I love to swap them out here on PBS and play in the games.  Gotta have paper books for that!

in about a year and a half, I have gotten about 200 free books (not public domain)....  but it does depend on if you are interested in what is being given away.  Of those free books about 5 replaced paperbook versions I had.  I do really like booksontheknob for finding free books, they give good descriptions of what is available instead of just a title & author.  inkmesh consolidates prices for many sites, so can give you ideas about who has books you are interested in available. 

I started downloading free books months before I got the Kindle, so I would have a nice collection and also to take advantage of the free books. I now have about 150 books. I have paid for some of them but not many, and only one over 9.99. The rest were under 5.00. I take into consideration, gas, time, the price of a credit, and whether or not it has a long wish list in deciding whether or not to spend money on an ebook.

I have had my e-reader for nearly 2 years and probably have over 500 books.  I also downloaded the kindle for PC and am starting a collection there too, although I don't like reading on my PC if I don't have too....so one day I may break down and get a kindle for those books or maybe when I get a new phone with a bigger screen I'll read on that.

I have only paid for 2-3 books.  One of them was $0.99 and another was $9.99 and was for my hubby. The rest have been free from the Sony site or other sites including smashwords.com