one timeslot on the air interface for GSM carries up to 22.8 kbps of ‘raw’ data in GSMK modulation (voice or GPRS), and up to 55kbps in 8PSK modulation (EDGE).
one user can get up to n timeslots when transferring data (GPRS / EDGE), depending on the class of the mobile. It is usually 4 timeslots… so the data rate can reach 220kbps in excellent condition. In urban environment, one user can usually expect 180kbps (outside busy hours).
for data TS allocation vs. voice TS allocation, it is a little more complicated and vendor’s dependent. Let’s just say that they usually try to put the data TS on one side, and the voice TS on the other side of the TRX, and the voice has more priority in case of conflict..