Kalimera Minimate
To inrease your bottom time you must try to inrease your sence mecanizm in your body .. You can dive in shallow around 4m.. an wait there ..and close your eyes..
if you start sense the water(sea water)..following start scaning your body.
i find out oneday the best thing directly concantrate diaphram muscle . i tried many things whatever you think. scan from toe to head or from head toe
you must find out what works for you it is the thing you searching.and it is the thing you need. .and be a very good friend with your diaphram.. Diaphram muscle spends more O2 , cos' it is red/white mixed muscle and after you fill your lung you also streched your muscle , too
i never done cardio exercise.. Because i do not know how to do.. but i do only running 1day/2days a week(i do not know it is cardio).. around 4km or 5km without stoping ..there is a hill side close to my house ..but there here many wild dogs and wild foxs below hill... it is so far from city center.. very clean and pure air.... Running makes my muscle hot(it ready to go!) i start streching around 10 minutes..
last summer my PB best was 7:39 min ..but after i broke my left leg i gave a biggggg break...
last week i tested myself after one week running and streching exercise .. how long i can hold my breath?.. i had great schock ..it was 4:00 min i had lost almost half of it..
i do only max hold after heating my diafram muscle 2 min or 3 min.. not directly going to max hold.. fisrt hold is till the first constarction.. second hold till the constarction+ 1min more... Last(3) max hold...it worked me before.. it will work again oneday.. i am sure
anyway the result was i can tell you that it was to bad ..it my body stoped me at 4:00min. it was not due to my 02/C02 level. .. it was .. i lost my mind control. everthing fly away.. i am afraid.. i thoug i am gonna get black out..i am afraid of when i entered second mode(dark side)
.... anyway my aim was to touch 10+ min.. But..i must re-start the engine botton!
There here many pro freedivers .. i bet you will learn more ..
As erik mentioned below many parameters effect your bottom time.. The real bottom time is the sign your body to return to surface ..
last word: i never forget the home made wines consist resin at KOS. it was so cheap like me who has no money

(one of Dodecanese Islands)
Cheers!
Ps: don' t ask me why my english is toooo bad?... I really do not know..