That the player is winning “decently” is not the only information needed. What people consider a decent win rate varies from game to game, and person to person. Winning 6 big blinds/100 hands is decent in a tough online HSNL game. Winning 30 big blinds/100 hands is decent in a soft online micro stakes game. The difference changes the bankroll requirements by a factor of 5.
Multi tabling does not intrinsically change the bankroll requirements. However, those who play 10 or more tables usually accept much lower win rates as “decent” than those who play 1-4 tables at the same level.
The standard deviation varies tremendously from player to player and game to game. Some full ring players have a SD/100 of 60 big blinds or less. Some shorthanded players have a SD/100 of 160 big blinds or more. (See the stats threads in the strategy forums.) This difference means a factor of 7 difference in bankroll requirements for the same win rate.
Bankrolls also mean different things to different people. If you can move down to a profitable game with lower stakes, and are willing to do that when you hit a downswing, then you can be safer with less. See the question by Gomes that I answered for an example where moving down can make someone as safe as though his bankroll were multiplied by 5.5. Of course, for some people (particularly those playing for low stakes), busting out is a temporary inconvenience instead of a disaster, which means that they can be far more aggressive.