How long before harvest are yellowing leaves ok?

Let's just assume a "typical" 63 day/9 Week indoor strain. How far in/how many days or weeks before harvest would you like to see (or at least not be worried by) some fan leaves yellowing?
I typically start seeing some around day 40 with most strains in my experience.
What all does everyone else think? I Assume some strains don't really yellow out as much if at all and others are more prone to it. I also assume it depends on when they run out of nitrogen which would depends on growth rate and feeding to a large degree.
Maybe it's a bad question due to ambiguity, but a question I was thinking about none the less. Thanks for your time, and hopefully responses.

Depending on strain, 42 days to 49 days is what I see. Some strains will start to yellow on their own, even before giving straight water the final weeks. Then some strains only yellow when given the straight water.

As long as you're not seeing yellowing before 6 weeks, I think you should be fine.

...i agree that 3 weeks before the chop is fine to see some yellowing.

...keep in mind that what media and style you grow in has some impact on this timing, meaning i grow in coco so i feed with every watering up to the final 2 week flush so i control when my plants start to yellow to some degree by when i start decreasing the nitrogen in the daily feed.

...with soil it can be somewhat controlled as well but through different variables that i wouldn't feel qualified to opine on.

...anyway, seeing some yellowing at day 40/63 is no big whoop and nothing to be concerned about in my opinion.

peace, bozo

The plant typically needs a touch of N after the stretch.

Sounds like that's what you guys are seeing. The week after stretch is completed the plant has to replace the N it burned up during stretch. If it's not provided by the soil or media, it will take from the leaves.

I normally feed 50% Veg and 50% Bloom first week of flower. Then at 3.5 weeks add a little veg nutrient to feed, just once. Week 4.5 begin any bloom boosters. Plant should yellow in last week after a week of flush.