For Bitcoin it is common knowledge that 6 confirmations suffices. For the hundred of Alt Coins however, I'm not sure what the best practise to be protected enough from double-spents and the likes is.
I saw Bter.com requires different confirmations varying on which Altcoin you want to deposit.
For example COMM-coin requires 10 confirmations, but BBQ coin 6, and Infinitecoin only 4.
Is there a mathematical method behind this? How do they know how many confirmations to require?