Learn French: Il travaille de cette manière depuis longtemps. - He has worked this way for a long time.

In French, when an action started in the past and is still continuing right now, you use the present tense with depuis (for/since). English uses the present perfect ("has worked"), which often tricks learners into trying to use the passé composé.

How would I change this sentence to say "He has worked hard for a long time" using the grammar from this lesson?

You would replace de cette manière with dur to make: Il travaille dur depuis longtemps. Even though dur is usually an adjective, here it acts as an adverb modifying the verb travailler. Because of this, it is invariable (it never changes its spelling to match gender or number).

Could I use comme ça instead of de cette manière?

Absolutely. Il travaille comme ça depuis longtemps means the exact same thing but sounds slightly more casual and conversational. De cette manière is just a bit more formal.

Why is there no article before longtemps? Shouldn't it be depuis un longtemps?

No, because longtemps is an adverb, not a noun! Even though English says "for a long time", French treats longtemps as a single adverb of time, so it never takes an article like un or le.