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>