Standby is defined as "wait and I will call you back.' If you reply, you haven't waited. I have just retired after 42 years mostly in towers and never wanted and usually didn't get a reply.
If instructed to standby, it is because the ATC is busy with something else and does not want to be interrupted. He/she will call you when able. ATCs have a number of prompts to remind them you have called, e.g. cocking the strip, so you won't be forgotten.