Will minoxidil help thin crown area after FUE hair transplant 18 months ago?

I do recommend that all my patients use some sort of FDA approved maintenance after transplant. This is for the native hair and not for the grafts, which are generally protected from the effects of AGA. Minoxidil, finasteride or low level laser therapy are all good choices and need to be used religiously for the foreseeable future in order to be effective. But it's important to note that NONE of these will regrown hair where the follicle has involuted (disappeared). They all do two things - they lengthen the growth phase, thereby preventing loss and they may encourage the hair to cycle into a more hearty "terminal hair" if it has started to miniaturize.
So, yes it's great that you've started using minoxidil 5% and should continue to use it at least once a day (twice may be better) and as for how long, you will need to use if for as long as you want to not restart losing hair (basically forever) If you feel like this is not for you then consider either finasteride (patients may have an easier time sticking with a tablet) or laser cap. Hope this helps