你哋明唔明白我？

點解同幾時你想學中文幾耐啦？
Does it makes sense to ask, "Why and how long have you wanted to learn Chinese" this way?

幾時--When

幾耐--How long

Hope it helps :)

會唔會，其實你嘅意思係：

你點解想學中文，同埋你想學左中文幾耐？

Do you mean:
Why do you want to learn Chinese, and How long have you wanted to learn Chinese?

你哋明唔明白我？

點解同幾時你想學中文？
Does it makes sense to ask, "Why and how long have you wanted to learn Chinese" this way?

Dun be lazy and please separate it to two questions. You can use 同埋 to join them.

點解你想學中文同埋你學左幾耐？

你哋明唔明白我？

點解同幾時你想學中文？ 點解你想學中文？你学學幾耐啦？
Does it makes sense to ask, "Why and how long have you wanted to learn Chinese" this way?