Well.....here's my opinion.
0.9% normal saline contains 154 mEq/L of Na+ and Cl−. It has a slightly higher degree of osmolarity (i.e. more solute per litre) than blood (hence, though it is said to be isotonic with blood in clinical contexts, this is technically inaccurate). Nonetheless, the osmolarity of normal saline is a close approximation to the osmolarity of NaCl in blood. So essentially, bacteria can thrive in that environment if introduced due to poor aseptic technique. However, the cold environment of the fridge (where you'll store it) will help to inhibit/slow potential microbial growth.
Now, if you were using it as a single dose, it wouldn't matter either way; but for storage, make sure you're using bacterial static water (which contains benzyl alcohol) if possible and keeping it in the fridge. This is the best way to ultimately prevent unwanted microbial growth.
You can get 30-60 days out your hCG (depending on who you talk to) after you reconstitute the powder. I've used it for 60+ days that way and still evinced appropriate effects from an anecdotal standpoint.