I wanted to know if saline injections are safe? And if so how long do they last in the buttocks?

Saline injections are not completely safe and do not last for long periods of time (likely hours). Serial injections may lead to hematoma, infection, or longterm changes. Raffy Karamanoukian MD FACS

Regards, 
Liquid silicone is unsafe, please do not use it ever. The autoimmune response could be triggered by this substance, if you look around on th Internet, you will find patients that have a lot of problems with this product. The inical use is for retine problems. 
Saline injections last one day. If they told you that it last longer, it is not saline injections, they are biopolymers or liquid silicone. The Biopolymers cause a severe inflammatory reaction, known as Iatrogenic Allogenosis and Autoimmune Syndrome (A.S.I.A. Syndrome), then the use of any substance that is permanent is not advised, it also could migrate at the inicial injection suddenly, via retrograde arterial flow, at blood stream to organs and even to the lungs; that would be a catastrofic event. In our plastic surgery center, I developed the technique of resection of biopolymers in face, lips, buttocks and lower back through video endoscopy, a surgery performed with minnimal incissions to treat patients with permanent deformities, secundary to biopolymers injections. You have to be checked by a specialist to know if it has affected the tissues or ir it caused you damage to the organism.
Please remember to be aware, do not let any permanent filler to be injected into your face, breast, buttocks, calfs, genitalia, etc. The safest way to increase buttocks is through your own fat injections or through silicone implants.
I hope the information was usefull, have a nice day! Dr Carlos Alberto Ríos.