like the others said...
i didn't look to see where you were and did see what prison you wrote to, but i'd say generally 2-3 weeks at most. out of 3 letters i sent at the same time a few weeks ago, only two responded... my first reply was about 6 days after i mailed them and the second response was about a couple days after that... and both of them were from the same prison.
so yes, i agree.. patience is key. i'm not a very patient person and have found it hard when expecting my next response letter in the mailbox and it not being there. but i pull through, do something to get my mind off it, and move on.
good luck though and keep reaching out! someone will surely appreciate what you're doing and hopefully reply back.
also, i don't know if this makes a difference, but in my first few letters, i made sure to include a stamp since i didn't know if they would have access to get them.
/Reurbo