I would tell people to definitely be READY to send care packages, but hold off until you can get some details on your country - it really depends on the country what is and isn't acceptable to send, as well as how long it'll take to get there.
For me, most packages seemed to show up between 2-3 months from when they were sent. I had one take 2 years, and a couple that never arrived. One of the other volunteers who had extended actually received a package she was sent BEFORE she left for staging just before she was ready to close-of-service in her THIRD year.
In some countries it is also expensive to RECEIVE packages, as you have to pay a customs duty or fee (aka bribe) to pick them up. Also, I believe some countries don't allow you to receive anything but letter mail during pre-service training.
In any case, almost every package I received was opened up by someone and in many cases something was missing. Usually something small, but I know of other people who were missing some electronics or other things sent over. I would talk to people from the country you've been invited to about how packages work there specifically.
Regardless of how long they took or how much of a pain they were to get, we always appreciated whatever we received.