A jungler can level 2 gank as soon as they finish their first camp which, with a good leash, can result in them showing up in lane as a little as 7-10 (15~ for mid lane) seconds after the laners comes in. Depending on where they start, a level 3 gank is usually around the 3:00 minute mark give or take a few seconds, sometimes as late as 3:30 if they started on the opposite side of the map from your lane.
This is why it's almost always a good idea to help your jungler find out where the enemy jungler is starting. That information gives your entire team a good idea of when to expect jungle pressure, and allows your jungler to plan his routes in accordance.