Liquids that keeps the lowest temperature for the longer time after being frozen I am working on a personal project, however I know nothing about chemistry and I would like to ask you guys for a quick question. I need to use a liquid for my project, which after being put into a freezer will freeze, and will be kept frozen for the most time when taken out of the freezer into room temperatures. 

Ideally I would like to know if there is a liquid which can freeze after a couple of hours, and when taken out from the refrigerator it can sustain a temperature of -2 -3 degrees for the longer period, in normal room temperatures. And most importantly it can me used again and again. 

Thank you so much guys, I will be looking forward to an answer on this one.
To maintain a stable temperature while heat is entering a system implies some phase change: that is, the substance is either melting or boiling, keeping the temperature the same, while some part is changed form solid to liquid or form liquid to gas.

Pure water freezes at 0 °C, but if another substance is added, the freezing point (f.p.) may be lowered. Ordinary table salt, $\ce{NaCl}$, can lower the f.p. to -20 °C. Calcium chloride, $\ce{CaCl2}$, is even more effective. Both are used for melting ice from roads, with $\ce{CaCl2}$ preferred in cold weather.

You might test various concentration to see what best meets your needs. Ideally, the solution f.p. should be a bit above the temperature of your freezer.