I've four amaryllis in a big pot inside in a sun room. One has a leaf, perhaps eight months old, that is starting to turn yellow at the tip. No big deal, just curious. How long to amaryllis leaves normally last?
I too have one that has been going for months, though it hasn't flowered this year at all. Mine tend to go about 8 or so months in the greenhouse, but at work I have one that has been in leaf continuously for years (again, no flowers!!). Maybe the forced dormancy is needed to make them flower, so we are loosing out by being so kind to our plants!! :-))
I have seen a splendid multi-crowned specimen with leaves and flowers both present. Light exposure did not seem particularly strong either. But it was getting regular doses of fertilizer. All horticultural varieties of bulbs with enlarged flowers need to have the nutrient levels kept up in order to perform fully.
Leaf duration seems to vary by variety. Most of the amaryllises in my yard have leaves essentially all year, though they may die back shortly before flowering.
Re:Addendum to OP I've a second question. Tangent to OP. If amaryllis leaf starts to yellow does it matter if it clipped off or allowed to run its course?
Plants are often good at evacuating nutrients and other useful goods from leaves that are yellowing but still functioning. So "wait until shriveled" is prudent.