Well, there is actually no need to keep a lookout. When the foreskin retracts is simply a milestone like the first tooth or the first step. As far as anything else goes, it simply doesn't matter and as opposed to those other milestones, it doesn't come on schedule.
You pretty much know when the first tooth is going to come within a month or two and when the first step is going to come and when they are going to say their first word or take their first step or become potty trained. However, the foreskin becoming retractile is far different. It may happen soon after birth or in the late teen years but it really doesn't matter when it happens. There is no schedule.
Two things have to happen before the foreskin is retractile. The opening of the foreskin is made up of non-elastic tissue in young children and at sometime in the future, the body will slowly replace this non-elastic tissue with elastic stretchy tissue that will allow the opening to spread to pull back over the glans. This is an invisible process and it will look little if any difference before and after. One day, it will seem almost magically stretchy when it didn't appear to be a few days earlier.
Also in very young boys, the foreskin is adhered to the glans just like the fingernails are adhered to the nailbeds and with the exact same material, the synecchia or epithelium layer. Unlike the fingernails, this tissue will disolve someday and the foreskin willl no longer be adhered. The only indications you may see that this is happening is ballooning where the foreskin puffs up while urinating showing that the foreskin has released but the opening has not become elastic yet or there may be an episode of redness and swelling that lasts a day or so that indicates there is slight trauma to the foreskin related to the release. It will look like an infection but will pass quickly. The child may also report some pain for a short while.
Retraction may be something that happens progressively over months or years or it may be that one day, BAM! It's retractile. Every boy is different and unique.
So for a short summary, watching for retraction is simply from a curiosity standpoint for the mother, not something that should happen by age xx or something is wrong. Basically, it happens when it happens and it may be something you will never see because you son may be a teenager when it happens and certainly won't be wanting to discuss these things with Mom.
Frank