Learn Spanish (Latin America): Ellos llevan mucho tiempo leyendo sobre la teoría y la práctica. - They have been reading about theory and practice for a long time.

In Spanish, when an action started in the past and is still continuing right now, you use the verb llevar in the present tense, followed by the amount of time (mucho tiempo), and then the gerund (leyendo). English uses the present perfect continuous ("have been reading") for this.

Could I say "han estado leyendo" instead of "llevan leyendo"?

Yes! Ellos han estado leyendo mucho tiempo is grammatically correct and means the same thing. However, using llevar + time + gerund is extremely common and natural in everyday Spanish to express how long an ongoing action has been happening.

Can I change the word order and put "leyendo" right after "llevan"?

Yes, you can. You could say Ellos llevan leyendo mucho tiempo sobre... Both word orders (llevar + time + gerund, or llevar + gerund + time) are completely natural and correct.

Why are there "la"s in front of "teoría" and "práctica"? The English translation doesn't say "the theory".

In Spanish, you generally need to use definite articles (like el or la) before abstract nouns or general concepts when discussing them in a broad sense. English drops the article for general concepts like "theory and practice", but Spanish requires it.