1 How long can I hold on?
hold on 不及物
insist on 及物后面要补全宾语
2 persist in doing sth/insist on doing/keep doing坚持做某事，是 keep doing , keep on doing 好啊keep doing和keep on doing都可表示“不断（老是）做某事”的意思，强调动作连续不断，每隔一会儿发生，这时两者可以换用。不同的是keep doing中的doing是动名词，作宾语。例如：John always kept (on) asking questions.约翰总是问个不停。News of successes keeps (on) pouring in.捷报频传。 keep doing还可表示连续不断的动作或持续的状态。keep on doing则没有这种用法。例如：We kept working in the fields in spite of the rain. 尽管下雨，我们还是坚持在地里干活。Why do the dogs keep barking?这些狗为什么不停地叫？所以一般不加 on 肯定是对的，加on 要看情况的