How much time do I get to read a card before I counter it?
Your friend made that up. There is nothing in the comprehensive rules about how long a player may retain priority. Once you've got it, you keep it as long as you like until you play a spell/ability or pass.

If you're in an official tournament setting, your opponent can call a judge if he thinks you're intentionally playing slowly. If you're playing at your friend's kitchen table, then anything goes. Of course, the host can always decide to kick you out of his house, so try to play at a reasonable speed.