Hmm, everyone seems to have a time for it......
It should be 2 weeks. This is when the "stretch" happens. The plant will use more N then need P to complete this growth phase.
The biggest reason is to not cut the N and raise the P is the plant needs the N and not so much the P yet. Those who don't ,, get a lot of early onset yellowing of the leaves by the plant getting too much P too early. The result is reduced yields/quality and a plant feeding off it's self too much/early......A nice side effect of this is not getting too much stretch and having closer node spacing....
There is an argument that says that at or just like the day before the lighting change. You should feed with a mix of Veg and bloom feed to give the plant a "notice" that it's time to flower by tickling the hormonal response with a bit of extra P&K...
The method goes like this......1/3 bloom to 2/3 veg for the total amount needed to make the feed......I know, that seems like an insignificant amount but, to the plant, it does appear to assist in a smoother transition into flower with again, less overall stretch and better internodal spacing.......You must still, not feed the plant with bloom for 2 weeks after this little "bump" feeding....
I suggest if you have more then one plant.....Try the "bump" and 2 weeks no bloom on one and the no bloom for 2 weeks on another and see for your self , what works better for you.
One week is to short and 10 days is close, but no cigar! 2 weeks, ok?
Of course you can avoid any yellowing and run perfectly green, healthy plants start to finish by doing a water only organic soil....It's not as hard as it sounds....It's cheaper and you do less time in the grow and of course, Natural....
Doc