Yes, mild stenosis usually sees conception within a reasonable window. Usually less than 15 months, but I wouldn't say throw in the towel or anything. If it's more of a mid-grade stenosis, I could see it taking a little longer, though I wouldn't wait much longer than 18-24 months before diving into more intense fertility treatments. I haven't know anyone that took longer than that to conceive naturally, but then again, most people move on to more intense treatment by then anyway.
If I can ask, what are you using for lead up to the IUI? how many follicles are you getting? and what has your uterine lining measured at at insemination? If you're getting a thin lining due to Clomid, of if you're only getting a single fully mature follicle, you might consider switching to injectable meds. IUI has a pretty low success rate with natural cycles, Clomid, and Femara. Roughly 10% in most cases.
If you want to keep trying naturally, my advice would be to give the sperm a little more time to get where they're going. Though it usually isn't helpful for most women to stay laying down for longer periods of time after BD, you may want to try it to give the swimmers extra time to make it through. You have to think about how slowly they'll have to progress through your cervix. Try to find a position where most of the seminal fluid stays in after sex and stay that way for at least an hour. It takes semen about 20-30 minutes to fully liquefy, and then the sperm need time to travel. You can always DTD right before bed, prop yourself into position for an hour, and then go right to sleep once you can get a little more comfortable. I don't think they've done any studies on doing this with stenosis, but when it comes to less motile swimmers, tighter cervical canals, and men with higher viscosity semen, laying down does seem to help, at least anecdotally. It would be worth a try in my opinion.
If for some reason you do end up ever going the IVF route- do remember that general anesthesia is a really good idea for embryo transfer if you have stenosis. Without it, you may feel pain, and (from what I've read) that can cause the uterus to spasm more than it normally would, which can lower success rates and increase the risk of ectopic. I would think for IUI though, that wouldn't be a problem since you want the sperm moving up towards the tubes.