They usuallly go on a case-by-case basis, depending on the Rules Enforcement Level of the tournament (hey, at a GP, for instance, you are SUPPOSED to have a certain level of expertise, if you don't, too bad) and even the player him/herself.
For instance, Antoine Ruel (I think, might have been Olivier) got disqualified from a tournament, because his opponent was playing with a pair of mirrored shades "clipped" to his T-Shirt. Ruel tried to look at it to get a glimpse of his hand, and a judge actually saw him do it. Ruel tried to lie himself out of it, but it didn't stuck. DQ, no prizes.
Kenji Tsumura, (again, might have been another player, it's been a while) which got a LOT of warnings (usually under the excuse that a situation might have gone wrong due to communication issues) ended up getting a 6-month ban for repeting pattern of bad behavior, the DCI has a register under the player's tournament history of ALL the infractions that player might have commited.
I made that one as such to be as straightforward as possible, so players would catch the cheating intention. However, for instance, if player B played REALLY fast to get a turn gone so the game can't be reversed back, it might ALSO indicate he saw that. And in such cases, a judge could even allow the game to rewind back to the appropriate phase, since judges have the last word on each situation.