I think you can experience bigger jumps (2 levels) when players are in really bad GS and more likely jumps up of 1 when they are respectable and above. Remember that GS has invisible sub-levels too so you could go from awful (3.1) to inept (4.9) and it would be a good week. I reckon you'll get to average in 2 weeks then 1 week for each level of improvement there.