How long after opening is it safe to use a bottle of eye drops?

How long, after opening, is it safe to use eye drops for ROP (retinopathy of prematurity) exams in the NICU (a nursery for premature babies)?

All eye drops have an expiration date, which refers to the shelf life of a drop which has not been opened.

However, eye drops, once opened and used, risk being contaminated with the normal bacteria that live on the eyelids. Be very careful putting the drops in the babies’ eyes—it is easy to contaminate the tip of the bottle with a touch to the babies' eyelids.

Most bottles of eye drops contain preservatives which limit bacterial growth but not entirely. The older the bottle, the greater the chance that it has been contaminated, and the longer the time the bacteria have to grow. A good rule of thumb is to throw away any opened bottle of eye drops after 3 months. So an unopened bottle of drops can be used up to its expiration date, but if a bottle is opened today, you should throw it away after three months, even if the expiration date is 12 months from now.