Depends on how much it is fed, and how large the meals are but i have hatchlings/juveniles that may go 8 weeks between sheds. If the guy knew for sure that it was going to shed at the time of purchase then it definately should have shed within 2 weeks of you buying it. If it has a stuck shed then it will most likely feel dry with abnormal skin folds where there shouldn't be any. By now it would be too late to soak it as the oils would have cleared up and the skin would have stuck back onto the snake like glue. You can have a try soaking it but you may have to wait until it goes into another shed and make sure it comes off (both skins, that is). Believe it or not, but i've had snakes not shed there skin because they've been too nervous to leave there box to get it off, even during the night!!
Of course there's always the chance that the seller was wrong and the snake wasn't getting ready to shed.