Could seguir + infinitive be used in a similar way to llevar here? Perhaps...
Sigo trabajando en este colegio tres años
= I've continued working in this college for three years
Or would that have to be 'He seguido trabajando...'
I also note the absence of a preposition (por, durante etc) before the time/period phrase in the examples. Is it not necessary with llevar?
Saludos