I've emailed my T about 4 or 5 times since I started seeing her in July, half of them was just setting up an appointment. One of them (non-appointment related) was immediately after a session. My T always responds.

I think it depends on your T's own practices and her schedule. Mine usually responds quickly enough (day of or day after), but once she didn't reply for a couple of days. She may have been busy that time.