Do BBL results only last up to a year? I read that after a year patients aren't too happy with their results as they lose fat?

The results from BBL are highly dependent of technique.  I use a system from Wells Johnson that efficiently transfer fat and the results are long lasting.

    I receive pictures or videos from former patients that show results 3 years and more after the procedure to illustrate longevity of result.

The final result of a BBL are dependent upon many factors and for this reason the results are variable.  An implant that is placed is for the most part predictable because it will not change in size or shape. Fat injection for a BBL depends upon the patient's body type, as well as how the fat is harvested, processed and injected and the patient's dietary and nutritional  status after surgery.  All of these factors will determine the long term results. 
With all type of fat injections it has become well recognized that a certain percentage of the injected fat will not remain. This may vary from 20-50%. 
Both butt implants and fat injection have advantages as well as possible problems and complications. A final decision  must take into account  the balance between these

I think that most women can get a nice result after one procedure, but they sometimes want more and get a second or even a third.  Not all fat takes and so usually after 2-3months after the swelling subsides the buttocks become a bit smaller.

When properly executed, BBL results are long lasting.  There is graft loss, but the fat that lives is there for good.  Weight loss can lessen the perceived results and occasionally a second round is needed, but that is not the norm in my practice.  Part of the problem is that people's expectations can be warped by false advertising with post-operative images that are either taken early in the recovery period and/or digitally altered.  When evaluating results, if you see bruising in picture or even worse, a surgeons markings, know that that is not a bona fide result.  Look elsewhere. 

In my experience of performing over 1000 cases of BBL since 2001 I have found that if after 5-6 weeks the butt has remained stable without any significant absorption of fat, then there is an excellent chance that the fat will remain.An if a second procedure is desired it can be performed after 3 months( if there is sufficient donor fat). But, in my opinion, there is no comparison in terms of the naturalness of the result between fat transfer and the use of an implant. Fat can be molded and customized with the feel being that of a natural butt. The complication rate of fat transfer is also less than with the use of implants. You should consult with a Board Certified PS with lots of experience performing BBL surgery Good luck!