How long have you been translating this text?

Since when have you been translating this text?

Since what time...?
можно использовать?

Yes, Darya, very good)

Since what time – думаю, ошибкой не будет, но на мой слух звучит не так естественно, как два предыдущих варианта.

Как ваши варианты?

What if you're being paid for the translation by the hour and the person who hired you wants to know at what time you started translating?

Or what if someone wants to time you in order to gauge how long it typically takes to translation a certain number of pages?

Or what if someone wants to know if you were still in the office when something happened?

Ещё одни вопрос, можно?

Том все ещё смотрит телевизор. Он смотрит телевизор весь день.-
Tom has still been watching TV. He is watching TV all day.

Или везде нужно использовать Past Perfect Continuous?

Uly, you're not wrong, and I get your point. I guess that would be obvious from the context two parties have while talking to each other. Or the asking person would explicitly ask for a specific time on that or previous day if the person being asked had replied only date.

Tom IS STILL watching tv. HE HAS BEEN watching tv all day.

Thank you all very much 🙈

Что он делает сейчас – Present Continuous.

Что он делает уже длительное время, которое указано, и продолжает делать это – Past Perfect Continuous.

Примеры Uly верны.

👍🏼