AeT improvement depends on two things primarily:
1) Your genetics. The distribution of adaptability is like a bell curve. Luckily, we humans are genetically predisposed towards endurance rather than speed and power. So, the bell curve for this has rather short tails. That means that while there are definitely fast responders and non-responders the vast majority of folks will fall in the middle with decent response to this type of training. Since you can’t control your genetics you’ll just have to find out by doing an experiment of one.
2) VOLUME: This the biggie and it is a universal rule no matter your genetics. For those with ADS the higher the volume of training you can handle just below your AeT (Z2) the faster the improvement will come. Caveat- DON’T TAKE THIS TO MEAN YOU SHOULD JUMP TO RUNNING 100MILES/WEEK. But the volume of this stuff is the main stimulus to improvement.
My TYPICAL observations: For those training under 8 hours/week of Z2 it is going to take 4-6 months to appreciably move the needle. Once over about 12 hours/week decent gains can come in 8-12 weeks.
I hope this helps.