a) “那我（可）就不客气了” can be mainly used in the 2 below situations. Example（1）： You come and visit a friend’s home, he has prepared a meal for you. He say:”别客气，多吃点.”（Help yourself.）,then you could reply :“那我（可）就不客气了”. Example（2）：You are angry with someone, and you don’t want this happen next time. You could say :”我很生气，下次我（可）就不客气了.” （I will teach you a lesson next time.） b) 怎么也得= at least 至少, you could say “至少（也/也得）有五个小时了” as well. Example : You have already wait a subway for at least half an hour, a stranger just come and wait behind you. He ask you : 你等多久了？（How long have you waited?）, you could say:”至少也有半个小时了”（At least half an hour.）But for a subway, half an hour is a really long time, here you want to express your impatience and angry, so you could say :”怎么/至少 也得有半小时了！”

a) 那我可就不客气了 is a threat. In English, someone might say "do that again and you may regret it". b) 得 is often used in the sense of "take", as in "the train takes five hours to reach Beijing.

a) 那 我 可 就 不 客气 了= but i'm not being over-polite in this very sentence 可 is not from 可以 (can) but from 可是 (but) b) 怎么 也 得 有 五 个 小时 了 吧。 = how come 5 more hours are needed? 怎么 = how come 也 = too, also 得 = děi = must, need

这些是地道的中文，要结合当时的场景和对方的语气来判断意思。而且不能每个字每个字的拆开来理解。

那,then. 我,I. 可就,meas a kind of "take for granted " 不客气了 meas not very polite or gente. 那我可就不客气了,means: then i 'll do something not very politely.