How strong are the brand contracts for players? Are they a one-season only thing or is every contract different?
Can a player buy himself or the brand by a player out of a current contract if he wants to be sponsored by a different brand?
In my experience they're 2/3 year deals usually, but it does depend on their age and prospects! With the IPL, tournament based contracts are eeking their way in, but not quite mainstream. I suspect some Indian brands may have signed a few players for the WC on a similar basis.
Usually they come to a natural end, the brand and player then re-negotiate or go their separate ways. I haven't seen players buying their way out of contracts, but that isn't to say it doesn't happen.