Once you send through InmateAid, the postcard has to go through the facility's mailroom process before it reaches your friend. Plan on about 6 to 7 days from the time you send it to the time it is in his hands. That is the typical window, though some facilities move faster and a few run slower depending on staffing and volume.
As for writing back, that depends on him. He can respond as soon as he receives it, but he will need to send his reply through whatever mail system the facility allows outgoing mail through. Most federal and state facilities permit inmates to send letters out, though they need stamps or funds on their account to do so. If he does not have those resources, that is something you can help with.
One thing worth knowing is that when he writes back, he will be using the return address from the envelope, which is InmateAid's Florida address. Make sure he has your correct contact information either in the postcard itself or in a follow-up letter so his reply finds you.
If you want to keep the communication flowing on your end, InmateAid makes it easy to send additional letters and postcards without your personal address ever showing up on the envelope. That privacy protection stays in place every time you send.