You don't usually have trouble with someone you never have trouble with, even after many years.
ーYou don't usually have trouble with someone you never have trouble with, even after many years. On the other hand, if there is someone you do have trouble with, you'll get into it with them in less than a week.
「何年たっても、揉めない人とは揉めない。反対に、揉める人とは1週間あれば揉めるだろう。」
You don't usually have trouble with someone you never have trouble with, even after many years. で「大抵、揉めない人とは何年たっても揉めない」といえます。
have trouble with ... で「…と揉める」と表現できます。
get into it with ... はここでは「…と揉め事を起こす」のように言えます。
less than a week で「1週間以内に」
ご参考まで!