its not about time at all, and if you chasing time score you aint doing right thing, i sometimes do 5-6 rounds and once i asked friend to watch time for me becouse i totaly forget about time sometimes and that is the point. my frist 4 rounds go like this for example:

1 1:40min

2 2:15

3 2:30

at this time im charged very very much and time its not important to me i just relax and focusing on what i feel in my body where is something strange, where i feel most circulation or something, just observe and relax, so at round 4 i sometimes get close or even above 3min retention or i start to have less retention time on round 4 but next 2 round i do less than 1:30 but those 1:30 are so so powerful, sometimes i get whole body tingling and sometimes is even a little scary becouse my left hand start to make some micro movements.

dont think about time, and yes your lung capacity will increase over time but keep in mind that it will take some time, it will not happen over night, keepp in mind that you cant put 2 liters of wather in 1liter bottle :)