he estado trabajando muchas horas últimamente

rule of thumb:
"to be" + [-ing verb] -> you must use "estar"    (NO "ser")

I am working -> Estoy trabajando

She was eating -> Ella estaba comiendo

Peter has been singing -> Peter ha estado cantando

They had been eating pizza -> Ellos habían estado comiendo pizza.

(PS. Remember that in Spanish we don't use "continuous tenses" to talk about future.)

he estado trabajando

he estado trabajando muchas horas últimamente

<em>To be= "Ser" o "estar", in this case, el verbo "estar".</em>