Depending on the size of the aircraft, very powerful. One way of thinking about his would be that the velocities in the wake are directly related to the lift produced. So, heavier the aircraft, larger the lift and greater the velocities.
A human being inside a vortex of a large would feel like he's inside a tornado. For all practical purposes, it is a mini-tornado.
The maximum vortex strength occurs when the aircraft is heavy, slow and clean (with low wingspan). Wind speeds upto 240-300 kmph has been recorded in certain cases.
This is the main reason for having separation intervals between two aircraft taking off, especially after a 'heavy' aircraft. The best examples of what happens when someone is inside these wingtip vortices comes from incidents where (smaller) aircraft gets caught up in the wake of (larger) aircraft and it doesn't make for very happy reading.
- In November 2008, a SAAB 340B got caught in the wake of an Airbus 380 conducting a parallel approach. During this incident, the ATSB report shows that the SAAB 340(which is not a small plane by any mens, having a wingspan of over 20m),
...experienced an uncommanded 52° roll to the left, in conjunction with an 8° nose-down pitching motion. Immediately after, the aircraft rolled through wings level to a 21° right bank angle. The aircraft also experienced an altitude loss of 300 to 400 ft in the 9 to 15-second period during which the crew regained control of the aircraft.
From these incidents, it is pretty clear that getting into the vortices, especially the large one is a very bad idea.