I haven't studied maths since secondary school.
I haven't studied maths since I was at secondary school.
Since he failed to get that job he's been moping about the house.
Since he didn't get that job he's been moping about the house.
Feels fairly informal to me. I'd avoid this construction myself because "since" has the alternative meaning of "because" and here I have a sense that I'm trying to understand it in that sense, failing, and falling back on the sense of "in the period of time from then to now".
Entonces no sé cuál es la regla que Perikles invoca, pero tampoco sé por qué lo ha mencionado, puesto que (¡since!) la frase de Irma no contenía la palabra "since".
Hace mucho tiempo que + verbo perfecto = Verbo perfecto + for a long time.
E.g. Hace mucho tiempo que soy profesor = I've been a teacher for a long time.
Hace mucho tiempo que no estudio las mates = I haven't studied maths for a long time.